Whrr.. that's odd.. I run a Radeon HD 3450 on my windows xp computer, it plays it around 100fps
The only problem is with my computers, for some really quite annoying reason I can't actually run the game above 1024x786 or it just crashes to desktop, which is quite annoying.
But eh i'm not complaining, I can actually play multiplayer flawlessly
My newer computer runs a Radeon HD 5700/5750 with no slowdown
.. also I see above, I posed that I gave up on radeon cards.. oops XD well the HD series seem to work for me now ^^;