Grun using a AMD 3500 A64 going from a 9800 pro to a x800 there was no noticeable difference in fps in AH.
AH is more cpu dependant currently so give it a go i do not think you will be disappointed.
Thats not to say there are no improvements from using the X800 series with AH but while using the 9800 pro i had a 12 fps jump on runways switching from a P4 3.2 to the A64 3500, so the cpu is where you will make most gains currently in AH.