Phoronix checks out whether the open-source Mesa driver for ATI cards in Linux is any good:
"With Mesa 7.1 having been released this week and the
open-source R600/770 3D support just around the corner, we've taken this
opportunity to see how the open-source Mesa 3D stack compares to AMD's
monthly-refined Catalyst Linux Suite with the fglrx driver performs for
the Radeon X1000 (R500) series. In this article are Mesa 7.1 and
Catalyst 8.8 benchmarks for the Radeon X1300PRO and X1800XL graphics cards."