Recently I got an X1900 XT so I could play oblivion better. I was getting FPS around 30 outdoors, which was great. More than double what I would get sometimes with my old 6800GS.
Now, all of a sudden, my frame rates are in the single digits outside, for no apparent reason. My card still scores almost 9000 on 3DMark05, and using F.E.A.R's built in test, it puts out 70 FPS. So it's definitely not the card. And for anyone who has played oblivion, I have no sound in the opening credits.
More aboutoblivion frame rate problems
Did you update drivers recently? It's possible there's a bug or an improper install.
Did you up the graphics in Oblivion with the new card? If you're enabling HDR+AA you'll most likely see a performance drop with that kind of eye candy.
Could also be a driver conflict--did you uninstall the old nVidia drivers, run driver cleaner pro, BEFORE installing the new ATI drivers?