if I turn on everything, and run @ 1280x1024 my machince cuts through it, with no lag at all. The fact is though, that while it is smooth and looks good the fps are easily under 50. I have modded the game to allow even better textures at distance and increased shadow and object distance (tweakguides.com is great) just to push my card as hard as it can...
...and it does. Oblivion is demanding, and the 1900 is almost identical in architecture to the chip in the xbox360, which the game was designed for.
as good as it is though, faces and lip-synching is better on HL2 (no self-shadowing but better overall), and the fact that the game does not use anistropic filtering at all is just retarded. w/o the mods i have used the distance textures look just dumpy. But once you unlock the stuff "hidden" the game looks stinkin amazing and the trees, grass and shadowing is why it is so taxing.
if you play Oblivion... which I do not see on the OP, but the link someone gave had that... then ati is the way to go. Not only is performance better but you get HDR + AA (w/ a patch) that Nv just can't do.