Especially for lunch, I would heartily recommend Sakana, on P St. just west of Dupont Circle. It is an unassuming little place, but prices are very reasonable, and the quality is high. I eat there probably twice a month, and it is always hopping, with a good portion of the tables speaking Japanese, which I always take as a good sign in a sushi place. If you order three rolls/nigiri/sashimi, you get miso soup and salad for free. Plus, if you want to break your "healthy food" sashimi kick, the Sakana Tempura roll (or as my friends and I call it, Junk Food Sushi), is possibly my favorite food ever!

I would also recommend Kaz Sushi Bistro - more upscale, and although known primarily for inventive rolls, the sashimi & nigiri is also quite good.

I myself have not tried Sushi Ko, which was recommended above (never find myself in that neigborhood), but I have friends who adore it.