If you're still interested I installed an ATI AIW 9800 Pro in the same system I listed above. I initially bought it for a 2nd system but I think for now I'm going to keep this in my main computer if the rest of the games run okay.
Stock core and memory since I don't know how to oc the ATI cards yet.
Icetomb map and just another rough estimate (spectate mode):
1024x768 4xAA and 4XAF: never saw it dip below 35fps and it seemed stay in the upper 40s to mid 50s most of the time. With the 5900U it seemed to say in the mid-20s and until turning of 3D hardware sound would also dip into the teens occasionally. The min of 35fps with the AIW was with hardware 3D sound + EAX still on.
Also, didn't do a clean install. Just used drive cleaner and installed the AIW after that. I was initially worried b/c my 3DMark03 score dropped to 5654 with the AIW 9800 Pro versus 6388 on the 5900Ultra (using the flicker-fix drivers). But, game-wise, at least with UT2003, it has performed much better. I haven't tried anything else but figured this might be a good comparison for you since I installed it in the exact same system I had the 5900 Ultra in.