Try installing the latest drivers for your graphics card.
+1 What Catalyst version do you have installed? GTA IV was running crap for me but then I installed 8.12 and WOW!
Not necessarily, the Brisbanes (65nm X2s) have been around for a while but the 6000+ Brisbane is a new addition. I suggest you get hold of your motherboard's manufacturer and get them to tell you, and make sure you specify it's the 3.1GHz 65nm 1MB L2 Cache
6000+ you're talking about and not the 90nm.