7900GT will not run 1024x768 max in Oblivion very well, it all depends on your definition of High. I think you will find Oblivion very playable at 10x7 on a 7900GT, but don't be surprised if you are tweaking grass distance, shadows, etc. because of some low fps in outdoor areas. Not sure in the end if you'd call that High details and be completely satisfied. Oblivion just hammers the GPU and for that game the X1800XT is the better choice.
All I have to go by is Firingsquad averaging 23 fps in their foliage test at 10x7 max, and Anandtech averaging 21 fps with a low about 12fps in their gate benchmark at 12x10 and their defintion of high settings. So clearly IMO both of those settings are above a 7900GT's playable range. http://www.firingsquad.com/hardware/oblivion_high-end_performance/page5.asphttp://www.anandtech.com/video/showdoc.aspx?i=2746&p=4