What part of DC will you be in?
Sushiko on Wisconsin Ave above Georgetown is one of the best, but it is pricey.
I've heard good things about Sushi Taro on 17th St, but never tried it.
Tako Grille in Bethesda (also on Wisconsin) is also very good.
In Virgina, I like Sushi Yama in Vienna. They also do good soba and udon.