The 7900GT acnalogy was for right now. The Xbox 360 hardware can handle a lot more than whats thrown at it. Oblivion for the 360 was made at a certain graphics level and the 360 plays it at that. The GPU of the Xbox 360, when utilized correctly, is capable of 2x the performance of the X1900XTX, so that leaves a lot of untapped performance.
A 7900GT is a 7900GT, and it runs Oblivion as well as it can. The performance of the GPU does not degrade, the quality of the games improves. The Xbox 360 can scale with those improvements for up to 3 years. A 7900GT will maybe keep up with the Xbox 360 for 3-4 months at most.