Actually thats is pretty much exactly the difference between the 5800 and 9800.
The 5800 has a slightly higher core clock, but slightly different single and Multi-texture processing. The 9800pro has more bandwidth and better AA. the difference in performance will get more severe the higher you crank the AA and AF.
But at 1280x1024 with 2xAA and 8x (ballanced) AF the 5800 should do just fine and humm right along.
All this stuff really comes down to personal preference and what settings you like to play games at.
Overam Mirage 4700
3.2ghz P4 HT
SIS 748FX Chipset 800mhz FSB
60Gig 7200RPM HD
Radeon 9600M Turbo 128 (400/250)
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