This beautiful and classy old city is a difficult place in which to think about nuclear war. Yet that is what American and Soviet arms negotiators must do here, from their meeting room with its imposing view of Lake Geneva and the French Alps.

The easier thing to think about in this city is money. The place reeks of it: banks on every corner, yachts, Mercedes, lakeside villas and apartments with high ceilings.

But perhaps the arms people can learn something from the money people.

Certainly some of the wealth of the city was accumulated by taking what might be called "prudent risks," making investments that pay off reasonably well but are not too dangerous.

That is not far from what is needed from the leaders of the world's two nuclear superpowers if there is ever to be a real halt to the atomic arms race and an end to what President Reagan last week called "the nuclear anxiety that has become such a conspicuous feature of public concern throughout the world."

The nuclear strategists and war planners who sit in the basement of the Pentagon and its counterpart in the Kremlin can come up with dozens of arcane reasons and scenarios that show one side or the other having an advantage, no matter how minute, in the atomic balance of terror which must in some way be countered. Clearly, there is safety in a rough balance of power.

But the overriding truths about this balance of power are that both sides already have overflowing nuclear arsenals; that strategic nuclear war, in which ocean-spanning missiles and bombers of each side attack the homeland of the other, is mutually suicidal no matter which side strikes first, and that such a war is on the extreme edge of the plausible military threats that either country faces.

It is a significant first step that negotiators are taking here. What the Reagan administration calls these strategic arms reduction talks, or START, are the first new negotiations on curbing long-range, nuclear-tipped missiles and bombers in more than three years.

But ultimately, the decision whether to take the "prudent risk" is a political one that can only be made by Ronald Reagan and Leonid I. Brezhnev, or their successors. It is only the leaders who can act boldly, toss aside the endless reasons why they should not act, and take on the ultra-hardliners, the worst-case planners, the pressure groups and military-industrial complexes that abound in both countries.

If they don't, sometime within the next two years or so, the technical momentum created by another new generation of weapons now being born in the laboratories will take over and another opportunity to stop will have been missed.

For outsiders who wonder if leaders will ever be bold enough to take the prudent risk, the guesswork is made more difficult by what is known of the two key players, Brezhnev and Reagan.

The Soviet president has been ailing for years, and it is hard to know if he is really in charge or how firmly he could act if he wanted to.

Reagan made thoughtful speeches in November and May on arms control as he launched the American plans for reductions in medium-range missiles in Europe and now strategic weapons. But Reagan still has not inspired confidence that he is comfortable with the subject of weaponry and strategy when speaking without a text. It is hard to say if he will spot the chance and grab the ring if an opportunity arises.

On the one hand, there are practical forces pushing both sides toward sensible steps. Theoretically, at least, prudent risks are not out of the question.

The United States is developing a whole new generation of missiles and bombers which would present a major new threat to existing Soviet forces if deployed. But those weapons will also cost billions and there is growing public pressure both here and in the United States for a halt to the arms race.

The Soviet Union, on the other hand, has far more severe economic problems than the United States does, and it has foreign policy problems in many places. Moscow probably could use a breather in its strained relations with Washington.

But the politics on both sides are staggeringly complex; creating the political conditions and atmosphere in Moscow and Washington that would make an arms agreement possible will be extremely difficult.

The Reagan administration has adopted a hard line toward the Soviet Union that is reminiscent of the 1950s. Reagan has made it clear that he regards the Soviets as America's enemies, and that no arms control agreement would change that fact.

The administration clearly has secured Moscow's attention.

American officials believe that while the Soviets may not like it, they understand Reagan's rhetoric, can deal with it, and are impressed by the fact that the president, despite his economic problems at home, has been able to keep the Pentagon's record new budgets from being trimmed in any serious way.

On the other hand, the administration's rhetoric not only has been tough, it also has probably been viewed as insulting by Soviet leaders. The president has not only said that the Soviets lie and cheat but he also has relegated Marxism-Leninism to "the ash heap of history."

Dimitri K. Simes, a Soviet specialist at the Johns Hopkins School of Advanced International Studies, has reminded Americans of how Nikita Khrushchev's boast that the Soviets would bury capitalism had a galvanzing and lasting effect on American attitudes about the Soviet Union.

Similarly, Simes recently wrote, the Soviet Union, no matter what its other problems, "has also tended to pull together and play tough when pushed around. The Reagan administration would be wise," he said, "to encourage moderation rather than belligerence in the behavior of America's principal rival," including the Brezhnev group and those to follow.

There is probably deep suspicion in Moscow, not only of Reagan but also of those around the president.

The Soviets, especially since the resignation of Alexander M. Haig Jr. as secretary of state, have to be wondering whether there is any prospect that a compromise position could ever emerge among the hardline officials who are now dealing with arms control in Washington.

The two sides are very far apart on their opening positions at the START talks and if there is ever to be an agreement there will have to be compromise.

The crunch may come when the Soviets challenge the United States to move off its opening position with a counterproposal of their own.

American officials suggest privately that there is a little give in the American proposals, but they are talking about allowing a few extra missile warheads or being a little bit flexible on how bombers and cruise missiles figure in the talks.

There is no hint of a major fallback position even having been developed.

The chief U.S. negotiator, retired Lt. Gen. Edward Rowny, has gone to great lengths to try to convince everyone that the traditional American impatience in such matters is a thing of the past and the United States will hang in for a long, long time to convince Moscow to accept Reagan's approach to arms reductions.

"We in the West like to play Pac-man where you put in the quarters and see instant results. This is not going to be that way," Rowny said.

Guided by the belief, in Reagan's words, that the Soviets have achieved "a definite margin of superiority" in strategic forces, the Reagan administration's initial proposals would require much larger cuts in Moscow's forces than in Washington's and would take the heaviest toll in Soviet land-based missile power, precisely where the Soviets are strongest.

This plan is certain to be rejected by the Kremlin. Besides being unbalanced, the Soviets argue, the American proposals play down bombers and the newest thing in weaponry, cruise missiles, both areas where the United States has an important technical edge.

So far, the only Soviet counteroffer has been Brezhnev's call for a limited freeze on new weapons that would stop America's ongoing modernization program in its tracks.

Since the United States is preparing to deploy new weapons to restore what it regards as balance, the Reagan administration understandably considers this an unfair proposition.

So, as usual, both sides have a clear idea of what they don't want. But both are still groping for realistic formulas that they might really accept.

This beautiful Geneva summer will doubtless be devoted to the preliminaries; next fall or winter, the world will discover whether the season of prudent risks is coming anytime soon.