hello? you dont have to upgrade anything, just overclock a little. i really dont think you will have to do much of that to run the gx2. certainly not for an 8800gs. gtx is questionable. i vote not.
i'd drop another gb or two of ram in before i went off buying a new mobo and cpu.
i suggest option 3:
get the best graphics card you can afford and 1-2gb of ram and see how you like it. even if it is severly bottlenecked (it wont be), you will see a massive improvement over your current set up.