Nothing, my 9600GT is more then enough for the games I play or plan to play over the next year. At that point, I'll re-evaluate.
I'm waiting to see that those monster 800SP units from good ole ATI are gonna do for half the price.....then I'll ebay the 8800GTX and buy whatever gives the best bang for the buck.
Just don't get your hopes up based on the number of stream processors, as ATI's seem to be slower than NVs. First off, 320 are reserved for physics, so the 4870 has 420 SPs available. Secondly, the HD3870 had 320 SPs, and it was about as fast as the 9600GT, which as 64 SPs.