A fresh approach to dinner theater - rather akin to the Shoreham's old Blue Room revues - is the novelty for Old Budapest restaurant on Route 50 just west of Fairfax Circle.

Producer-director-performer John Moran accents a prime but elusive quality - professional gloss. All the excellent performers are familiar through previous leading roles in area theaters.

M. Taba's Old Budapest boast the atmosphere and food of his native Hungary, a large room, seating 250 at well-spaced tables. Instead of the usual dinner theater buffet, there is table service with menu choices. The staff is ever alert to the performers who scurry, skitter or saunter all over the place during their numbers. During dinner the company performs three separate sets and the busy four-piece band continues for dancing during and after the performance.

Moran aptly titles his revue "The Celebration," a neatly woven skein of musical show and film favorites and surprises arranged by Hampton King, also the arranger for Harlequin Dinner Theater productions.

Moran has devised a fluid format and King, smooth arrangements, familiar songs flowing into others via key words: "Sing Our Song," "Song in My Heart for You," "You'll Be Swell."

Smartly dressed, the performers all have their special turns, with the three women all different types: Jean Anne Kain, cool, blond and silvery, Pam Bierly, russet-haired, perky and belting superby; Beverly Cosham, dark, low-key and smoldering.

There are surprises, too. David Troup, a former Singing Sergeant has the rolling sound and humor for Rossinis "Largo al Factotum." His partner for "The Fantasticks" excerpts, Larry Backler, sings, and well, "Why Can't I Walk Away."

That song is one of the many in this format that prompts table games. Remember it? It's from "Maggie Flynn," which is hardly a footnote in musical theater history but should be - because of its score. Trying out at the National, it had book, music and lyrics by Hugo Peretti, Luigi Creatore and George David Weiss. Shirley Jones, Jack Cassidy and Sybil Bowen were in it, but it mustered only 81 New York performances the fall of '68.

Moran has a refreshing dinner theater idea, a format that can be endlessly altered especially with such savvy performers as these.

Dinner is at 8, phone reservations at 591-2800.