Your X1800XT should be able to play COD2 at 1600*1200, if you use 2xAA and 8xAF.
Oblivion will have to be set at 1280*1024, though. Even the X1900XT series can't run Oblivion at 1600*1200. You might be able to play it at 1600*1200 if you turn off all the shadows, since in today's games, shadows take up alot of performance.