ShaderMark 2.0
The PRO is anywhere from 1.25X to 2X faster than a Radeon 9800 XT in raw shader throughput, as measured by ShaderMark 2.0. Having only 3/4 the pixel pipes of the X800 XT, as well as being clocked lower, accounts for the performance delta there.
Impressive raw performance from a £300 (at the time of writing) card, as 9800 XT pricing stays somewhat static (again, at the time of writing). The cash difference buys you a value for money performance increase in raw shader power.