An upgrade from an 8800GT OC to a 9800GTX or GTS 250 is going to be a marginal upgrade. If you're going to spend $150, look for a used 4890, or even a 4870 1gb, which would run under $150, but would offer a more noticeable upgrade from an 8800GT.
(I actually have a BFG 8800GT OC AND an EVGA...