The only thing worth considering is to pop in new 6300 mobo and go from there. But the cost for this option outweighs the gain a new mobo will cost around $400 and then the CPU another $400 - then the FX3600 (that's if you can find one) all up another $500 - 800 depending on where you find the darn thing total cost $1,600 - and you still only end up with only the 8800gts not gtx.
I would actualy do that if it was cheaper.. I think I'm gonna settle with the 7900GTX + overclock. The extra memory will definitely make a difference.