I had an XFX 7900GT card that went bad and I RMA'd it back to XFX. They have offered me an 8600GTS 256MB in exchange for no cost, or I can get a 7950GT 512MB for $20. I've been looking at reviews and looking for comparisons, but a lot of the things I read are conflicting. Here are some details on my setup:
 
Intel Core 2 Quad Q6600
Crucial Ballistix 2 x 1 GB Memory
MSI P6N SLI Platinum Motherboard
Antec TruPower Trio 650W Power Supply
Windows Vista 32 bit
 
Any thoughts or advice on what to get?

7950GT > 8600GTS (Both the 512 and 256MB variety) -

http://www23.tomshardware.com/grap [...] &chart=275

*edit* If you're in the market for a new card. The 8800GT 256MB just game out for ~$200 and scores pretty close to the 512MB version until you get into higher resolutions.

Pay the $20. The 7950 GT is notably better than the 8600 GTS.

If u can the go for 8800GT, otherwise a 7950GT is alot better than 8600GTS