More in a continuing series of columns on Individual Retirement Accounts: Where should I stash the cash?

Now we come to the critical question. Since passage of the Economic Recovery Tax Act of 1981 (ERTA) there has been an increasing clamor of voices seeking a piece of the IRA market.

No wonder. Although estimates of the amount of money that will wind up in IRAs cover a pretty wide range, it is likely that total investments may reach into the tens of billions of dollars a year.

That isn't as unreasonable a figure as it may seem at first reading. Five million workers (out of a total work force of almost 100 million) each investing $2,000 will generate a $10-billion nugget each year.

That's quite a market, and maybe it helps to explain why you've been beaten about the head by newspaper and magazine ads and TV commercials from all kinds of savings institutions, insurance companies, mutual funds and brokerage houses.

In fact, about the only sponsor of an IRA product that hasn't been making much noise has been Uncle Sam. Yes, that bearded gentleman has a ready-made IRA plan in the form of U.S. Individual Retirement Bonds.

The bonds come in various denominations (like savings bonds) starting at $50, and may be bought in person or by mail from any Federal Reserve Bank. This is about as safe an investment as you can find, but currently it pays only 9 percent a year, compounded semiannually.

Before we look at the other investment media you can use as an IRA vehicle, let me point out that there simply is no single one that is "the best."

Which vehicle is best will vary with the individual and will depend on several factors. The first is your age: As you get closer to retirement time, you should be more conservative.

A younger worker, on the other hand, can afford greater risks, because there is time to make up for possible losses and because there is a longer time span for speculative investments to mature.

When talking about risks, a very important element in the selection of an IRA vehicle is your personal temperament and investment philosophy--what has been called "risk tolerance."

If you are uncomfortable with risk and tend to equate the stock market with gambling, then you should be in a conservative or even an insured program regardless of the other factors.

How large a part of your total retirement program will the IRA be? If you expect to depend for retirement income on Social Security and your IRA, with perhaps a small amount of savings, then you should be conservative in selecting the IRA vehicle.

But you may have a secure job with a company, or perhaps with a federal, state or local government entity, that has an excellent retirement program. Or you may already own substantial assets that promise a source of retirement income.

In either of these situations, where your IRA is expected to generate only a small part of your total retirement income, you can accept the greater risk that goes with speculative investments.

A couple of other important points should be made here. The IRA is a tax-deferred investment program, so it may not make much sense to put your IRA funds into an investment medium that is a tax-sheltered program, unless there are other compensating factors.

Second, when you retire and begin to withdraw funds from the IRA, all of the withdrawn money is counted as ordinary income--and taxed that way.

So you should probably stay away from tax-free investments like municipal bonds, because you would then be converting nontaxable income to taxable income.

Perhaps not as obvious: Avoid investments that generate long-term capital gains. Only 40 percent of such gains is subject to tax; but if it is part of an IRA investment, it will lose its identity and be taxed in full when you start to withdraw funds from the account.

And finally, when we talk next week about the various kinds of investments available for IRAs you will find collectibles missing from the list.

Collectibles such as gold, works of art, stamps and coins were no longer eligible for IRA accounts after Jan. 1.

If you had an IRA in 1981 or earlier with investments in collectibles, you may retain them in the portfolio without penalty. But they are no longer valid for new investments for either new or existing IRAs.