Go for either 7900GT or X1800XT. Both are in the $300US price range.
As for a brand, for nVidia go for eVGA. For ATI look for HIS, Sapphire. I wouldn't touch Asus.
I agree :-D
If you want OpenGL performance and non-windoze compatibility go with nVidia.
If you want D3D performance and windoze only go with ATI.