"I ASK myself how I kept it all together all these years, and I just can't tell you. Anyhow, it's like a dream come true."

Millie Curtis is talking about her collection of dresses, hats, purses, shoes, fans, boas and rhinestone gewgaws, thousands of them, which already threaten to burst out of Overflow II, the Georgetown Museum of Antique Fashions, launched about a month ago at 1610 Wisconsin Ave. NW.

She is into her subject now, and her soft North Carolina voice goes on a mile a minute as she surveys the amazing display lining the walls, gracing the manikins, marching up the staircase to fill the entire upper story: a sprightly low-waisted Chanel number with superbly crafted floating panels, dating from the '20s; an 1810 wedding gown with beaded satin flounced skirt; elegant and immensely valuable handmade lace from France; a couple of rare early Fortuny dresses; a stunning gold Empire gown owned by Natasha Rambova, one of Rudolph Valentino's wives; Trigere frocks from the '40s, vintage Diors with the labels tucked away in seams to thwart thieves, early Balmains, Cardins, Charles Jameses, de la Rentas . . .

"I've never been to a yard sale or an auction. I got them from the church--I belong to three churches, actually--or people gave or sold them to me. The very first was this one, it came from Evelyn Walsh McLean when she died, and a friend of the family gave it to me in 1938. That was the beginning."

It is a handsome black satin flapper dress with huge rope pearls and a flame design in rhinestones. Curtis rents it quite often. Her daughter even wore it at Halloween when she was a little girl. It was built to last.

Other dresses and hats came from the likes of Alice Roosevelt Longworth, Robert E. Lee's family in Richmond, at least 500 from the estate of Mrs. Robert Low Bacon ("That was the icing on the cake!").

Some things are for sale or rent at the museum--the entire basement is given over to the costume collection of the late Jack Mullane, a costumer here for 50 years before his death in 1952, and those are all for sale--but many items are strictly to be looked at.

"I want to do fashion shows in nursing homes, things like that," says Curtis, and already she is getting lucrative calls for her stuff. Geoffrey Holder, the dancer, was in the other day and talked her out of two hats that were supposed to be for the museum. "The next day his brother, Bosco, came by, and he started looking at hats, too, and I said, 'Don't ask. Don't even think about asking. I already gave away too many.' "

There is a roomful of men's clothing, from gold-braided swallowtail coats with satin knickers and fore-and-aft admiral hats for presentation at court, to uniforms, collapsible tophats, authentic Borsalinos, Stetsons and private-eye snapbrims. And, of course, conventional formal wear.

"After my husband died (he was Dr. Howard Curtis, a radiation biologist at NIH) and my son and daughter went off (they have careers in Colorado), there I was, all alone in Chevy Chase with nothing to do but bowl at the Kenwood Club. So I started this museum. I can't believe I've done it. It seemed like a dream, all those things I've collected for so many years. Now I can't wait to get down here every day."

Just look at this one thing, on this whole wall covered with antique hats, look at these feathers. You wouldn't believe that many feathers. Everywhere feathers: ostrich, pheasant, dove, quail, heaven knows what. On the band, on the crown, drooping over the brim, sticking up like a fountain, swirling around like a whirlpool of rainbows: They're all into feathers! Did you ever know feathers were that big? . . .