I'm kinda the same way as I really enjoyed MW but had a difficult time getting the 8500 to run it good. I moved to a 9500 Pro and the game was great. It only got better as time went on. I hope Oblivion is the same + better.
I may try it with the two GS's in SLI but more than likely will go with my GTX for serious gaming.
Go with the 7800 GT. That way if it is a little slow at max settings you can just get another GT.