For the price range you list an nVidia GeForce FX 5600 would be your best shot.
You are correct about the AMD/ATI conflict problem but only partially . . . the problem is with the Via chipsets most commonly put on AMD boards. Via has acknowledged the conflicts with 4 in 1 drivers and ATI Catalysts and you can see the innumerable posts about it if you care to view the Via Arena support forum. Quite possibly, problems with ATI cards and Via based motherboards are the most common help topic posted on the Via Forum. ATI cards can be made to work, but generally nVidia seems to be an easier install. Here:
http://forums.viaarena.com/
As an aside, some folks actually have better luck with ATI graphics cards on AMD boards with nVidia chipsets. Sounds like a contradiction in terms but that is sometimes the case. Jim