My PC has an ATI 9250 SE 128 MB and its fine for SHIII. I also run Reflex XTR model flight simulator that features photoreal backgrounds and detailed models
And this runs at 90 - 100 fps on my Athlon 2800+ running at 2.2ghz with 1gb of DDR PC3200 ram.
The problem is that with a lot of cheaper graphics cards they sync the display to the refresh rate of the monitor to reduce the loading on the CPU, this is why more expensive cards work better as the video processor takes more of the load as they don't use V-sync so much.[/i]