The Hunt brothers proved the old adage about not laying up your treasure in silver; all of Washington seems to be investing in real estate. But not everybody. Some people are putting their fortunes in well, in baseball cards. If you don't believe it, just ask Mark Lewis.

"Card collecting is the best investment," trumpets Lewis, who publishes Card Prices Update, a monthly catalogue, on Long Island. "Better than gold, or silver or stocks."

Anyone lucky enough to own a 1910 Honus Wagner card, "the king of the hobby," according to Lewis, can hold on to it and hope its value increases.Or sell it now for $15,000.

But that's a rare example. George Brett is a more common indication of what's happening in the cards. Before the 1980 season, a card from Brett's rookie season was worth 75 cents. Then came his run at batting .400, and quicker than a line drive into the second deck, the value soared to $7.

The idea is to spot the budding George Bretts, or Jim Rices, or Dave Winfields (whose card value increased from $1 to $3.50 after signing with the Yankees) early. "Instead of buying one card, people will buy 500 at 30 cents apiece on the prospect that it will be a major superstar," Lewis says.

So let's say the old shoebox in the attic is still there, still full of baseball cards. Where do you get your collection appraised, and perhaps swing a deal or two?

There are auctions and private collectors, often found in the classified ads; and there are conventions, in hotels in places like New York and Philadelphia, where collectors and dealers spread tables with their wares, and check out yours.

Then there is Gerhardt's World of Sports Memorabilia, tucked in a nook of the Crystal Underground shopping mall in Crystal City. Cards are not the main business; nostalgia is. Baseball cards. Programs. Year books. Press guides. Magazines. For an investment, take a chance on a 1954 Hank Aaron (market value, $175). For fun, take a look at a 1956 Washington Nationals Senators) yearbook ("Nats' New Faces of 1956": Lyle Luttrell, Connie Greb, Libert Clarke).

Leroy Gerhardt, the owner, used to sell antiques and coins as a sideline; he had a few cards, and noticed that they drew attention. Then he got a few more, until he was splitting his business between cards and coins. Soon the coins were gone.

"I just started moving more to cards," he says. "I just happened to be in the right situation at the right time. Everybody's become interested."

Gerhardt formerly was an engineer with the department of energy. "I liked this better," he says. "This got to be a fulltime job, keeping up with the buying and pricing of cards. I figured I could make a go of it."

And how's business?

"I've been doing pretty good," Gerhardt says.

He has with customers such as Scott Kidwell, 10, and his father, Lynn Kidwell, of McLean. One recent Saturday -- crowded in Gerhardt's close quarters -- they bought a 1963 Mickey Mantle and a 1969 Mantle. Total prices: $35.50.

But the Kidwells aren't in it for the money. They are, simply, collectors, and Scott happens to like Mickey Mantle. Lynn Kidwell had some old cards laying around. "By a miracle of luck, they didn't get thrown out," he says.

"I started to look at the cards and I just got interested," Scott says. That was six months ago. Scott estimates his collection today at 3,000 cards.

Tom Carroll of Arlington was trying to get a peg on the value of old Senators' programs from the 1950s. Some even had the scorecard filled in -- 1958: Senators 4, Tigers 3. How long ago was 1958? The program lists these prices: soda, 15 cents; hog dog, 25 cents; peanuts, 15 cents. Enough.

"Will I sell them? It's hard to say," says Carroll, a regular at Griffith Stadium as a boy. "If I find out they're really valuable, I probably would. If push comes to shove. But I'd prefer to hold on to them."

Especially this time of year, with spring training still weeks away. Says Mark Lewis, who plans to open a memorabilia hall of fame, complete with Babe Ruth's underwear, "It's the next best thing to playing baseball."