The trouble with doing something beautifully is that the disappointments stand in such vivid contrast. The Prime Rib is so stylish, with its black walls discreetly gold-trimmed and its lucite-topped piano, that you doubly resent being crushed into the doorway that serves as a waiting room. Elegant tables with brass lamps refecting the black-and -gold rimmed china are so small that the waiter has to ask where he can put the bread basket. It is as smoky and noisy as a poker den, masking the pleasant dinner music. Though the menu is limited to three steaks, roast beef, lamb chops and three seafood dishes (which you could justifiably save for another restaurant), it is enough, for the beef is superb, well aged and carefully cooked, served in gargantuan portions. Appetizers - seafood cocktails, snails, clams casinor and lobster bisque - are presentable if not memorable. Except for the buttery crisp potato skins, the vegetables are not more than pleasant. Desserts, too, include only one of note, a tall goblet of cold buttered rum, which tastes like melted ice cream with rum and nuts, a brilliant idea. If you enjoy what Prime Rib has to offer - handsom room, prime location, and the best of the beef that made America famous - and don't expect it to act Continental, the restaurant is well worth the money - as long as you have that much to spend.