An old standby among Japanese restaurants by now, Nara remains as fresh and up- to-date as ever, its dining room of wooden booths and sushi bar still pretty and clean looking, its menu concentrating on sushi. There are tempura, teriyaki and sukiyaki, as well as rice and noodle bowls to round out the selection. But most of the menu is raw fish -- as appetizer, as main course, as multicourse dinner. The tempura is not as light and crisp as it might be, but tempura oysters are particularly good because of the quality of the oysters rather than their batter. And the choice of sushi is not wide -- we couldn't get salmon skin rolls, for instance -- but the fish is in good condition and the sushi bar is staffed expertly. More important, you can choose sushi by the piece as well as in pairs, the prices are modest, and you could dine on an array of little dishes -- raw beef sashimi, flounder sashimi, bean curd or vegetable appetizers and sushi- for a rather inexpensive dinner in most agreeable surroundings with service to match. 7756 Wisconsin Ave. NW. 986- 9696. L $4.50-$8.95, D $7.50- $13.50. L daily ex Sat, D daily. Closed Sun. AE, DC, MC, V. Res sugg. Full bar.