HEXUS posted a review on the ATI Radeon X1900 XT and XTX

R580, moreso than any modern GPU that meets the D3D9 spec, therefore relies on software to show it off. Even current synthetic benchmarks designed to show off theoretical rates in 3D hardware can have a hard time exploiting the tripling in fragment processing ability. That's not to say the performance increases at the same clocks as R520 are invisible. Clearly they're not without increases, especially at the higher resolutions, of up to 30% in the games we tested, clock-for-clock.

ATI Radeon X1900 XT and XTX Review