None of these are great, but they're the best I've found in Manhattan:
J&R - downtown across from City Hall Park
DataVision - corner of 40th and 5th
PC Mania - North side of West 14th just west off 6th Ave.
The
Marketpro computer shows all over NJ and NY on weekends have things you won't find at CompUSA, etc.
It's cheaper to order online, though.