I have this....... http://www.newegg.com/Product/Product.asp?Item=N82E16814130033 Oblivion with everything on max will not run 100% - it lags. Will the X-1900XTX get the job done?
with those specs at that res you should have it running great. i have a lowwer speced rig with higher res and i am happy with how that game runs but i dont have it all on full. i have aa + at @ x2, bloom on and some of the shadows and draw distances down a bit(self shadows off) and it looks great at sunrise. check some oblivion forums and see if you need a patch for dual core cpu as some games do. and imho oblivion will punish every gcard thats out at the min in one way or another.