Dunno if anyone's still having problems, but I found even with a 9800GTX that with antialiasing enabled I had a horrible slowdown (from 70fps down to 6fps sometimes!) when Antialiasing - Transparency is set to Supersampling in the nVidia control panel.

It affects the antialiasing on leaves and foliage in particular, and is enabled when AA is set to 2x or more in the Oblivion settings. Supersampling hammers the graphics card, and gave me 6-15fps when looking at a lot of trees (looking behind you just after coming out of the sewers for the first time was a good test). Multisampling gave me a steady 50fps when looking at the same trees, Transparency AA Off gave me a steady 70 fps, regardless of where I was looking.

Image quality doesn't suffer much, particularly if I'm using multisampling, and though I haven't tested yet, I'm sure Stalker suffers from exactly the same problem.

Hope this helps... Hopefully this thread topping the google list fo "Oblivion Antialiasing Framerate" should get the information to the people who need it! :D