I have an ATi All-In-Wonder 2006 Edition (Based on the ATi X1300. Also, non-OCed.), Catalyst 6.6, and U20. Graphics all maxed out ingame, no AA, no AF. In fullscreen i get ~60FPS (with jumps to 80-90FPS in singleplayer with no AI), which can drop to about 30 with a full grid of cars in front of me.

However, my problem is that in windowed mode, I only get 6FPS or so, constant. This can be really annoying when I'm talking to friends while testing a set, etc., and I use windowed to be able to easily switch from LFS to AIM/MSN. (And just to answer the question that is sure to come up at some point, it is not the IM programs that lag it. I can run them + WMP + Photoshop at the same time as LFS, and it runs fine in fullscreen. It's just windowed that is acting strange.)

Besides AIM/MSN, the only tasks I have running are AVG, ATi control center, and windowsblinds, plus essential windows system programs. On an interesting note, I had this same problem in Q, but unloading windowsblinds fixed it. However, in U20, this fix doesn't work.

(Other system specs if you want them: AMD Athon 64 3800+ 2.4gHz, 1MB RAM.)