I have the GT solution and I run out of mem sometimes when quaduple buffering and 8x FSAA, same res. I think SLI GTO is the best solution because it has 512MB ram, well plus it's faster than GT's and same price. about $500
For just oblivion, since ATI owns oblivion, they would be about the same. Overall though 7900GTO SLI would win more than X1950XTX.