9600 GT is approximately 80% of the performance of the 8800 GT for about 80% of its price.
I have seen 9600 GT's on sale for below $150 on many occasions.
If you must absolutely purchase a card now, I would first look at your LCD's native resolution. Identify the top 3 games you will be playing at least 80% of the time. Use various online hardware review sites to identify whether the 9600GT can provide you with sufficient performance at your native resolution. If the answer is >60 FPS most of the time, then spend $150 and purchase the 9600 Gt. If you require the 8800 GT to get >60 FPS most of the time, then purchase the 8800 GT, but I'd like to recommend you get a brand like EVGA to "step up".
If your goal is playing a game like Crysis on high settings at 60 FPS, or if the 8800GT 512 does not give you >60 FPS at your monitor's native resolution, I may wait another 2-3 months for the next batch of cards from our favorite Red and Green team.
The reason being, is: ATI is on-track to releasing their next-gen Radeon HD4870 by this July. NVIDIA will likely follow suit with their answer to the HD4870. Many will agree that the jump from 8800 GTX to 9800 GTX has not been as significant as the jump from 7800 GTX to 8800 GTX. The new batch of cards just on the horizon promises to be a significant jump. I would hate to see you commit $250 and be stuck with a video card that cannot provide you with the frame rates that you desire...