Oldblivion
At first I was pretty sure it was too good to be true, but then I had my friend (with an FX 5500) try it out, and he now gets 16-28 FPS in forests and no lag.
What Oldblivion does is replace sm 2.0 with sm 1.1 and it makes a BIG difference with the framerate.
The water, unfortunately, looks pretty bad with Oldblivion, but that's a small sacrifice; the other graphics look fine.
If you're not convinced, here are some screens taken on a comp with a GeForece 3 and 512 RAM:

