Theres a new nforce 2 unified driver out now. It includes a kernel patch for nvidia GART support, I think that means you can now use your Radeon cards accelerated with an nforce 2 board.
CP
<edit> Just looked, the patch is only for 2.4.20, although I don't know if you can apply the patch to 2.4.21 after applying the nvidia patch</edit>