your choices are limited my friend as the best sushi places are in the galleria area and westward. the closest would be "the fish" in midtown (5 min drive), a trendy sushi restaurant with decent sushi and food.
the other good choice would be "osaka" on westheimer near montrose (10 min drive) a couple more miles away. i'm not wild about this place either, as it can be hit or miss. a lot of people love it though, but i think it's because they bring you free gyoza and ice cream and the service is pretty good.
no matter what anyone tells you, do not go to coco's, yan, or zushi. all terrible places. also, near kirby and richmond still farther away from downtown (but closer than the galleria area) is the sushi trifecta: miyako, sushi king, and cafe japon all within a block away from each other. SK is the best of the worst, but my advice is to skip them all.
if you are willing to go a little farther, i would then consider kubo's in the village (20-25 min depending on traffic) or nippon near the museum district (15 min). they are both japanese owned (none of the previously mentioned restaurants are) and are good to very good.
sage 400 and kaneyama are my favorite places for sushi in the city. sage 400, a modern sushi place w/ japanese fusion cuisine and excellent sushi, will be a ~20-25 min drive depending on traffic. kaneyama is pretty far out west and offers more traditional sushi and japanese fare.
309 Gray St Ste 107, Houston, TX 77002
Osaka Japanese Restaurant
515 Westheimer Rd, Houston, TX 77006
Nippon Japanese Restaurant
4464 Montrose Blvd, Houston, TX 77006
Kubo's Sushi Bar & Grill
2414 University Blvd Ste 200, Houston, TX 77005
Houston, TX, Houston, TX
Kaneyama Japanese Restaurant
9527 Westheimer Rd, Houston, TX 77063
What neverfull said is pretty close to what I would say. Sage 400 is my current favorite in Houston. Osaka would be my second choice, with Kaneyama being a close third. After that, Nippon, Kubo's, and Azuma would be a tier below. Azuma is in the Rice Hotel; very close to the Hilton. It's not as good as the one on Kirby, but it will certainly do in a pinch and is quite close.
Thanks for the suggestion on Kaneyama. I am visiting from DFW and was really pleased with the meal. The Kaneyama roll was very good (yellowtail, eel sauce and tempura flakes) and the Caterpillar (eel roll w/ avocado on top) was even better. I am in town for at least a couple more days and plan to go back. Thanks!