Microsoft has released a hotfix to improve performance on dual core systems. It fixes a possible decrease in performance during demand based switching. Cool 'n Quiet must be enabled for the fix to work, and Microsoft will only send you the hot fix if you have been experiencing the problem. The issue is how do you know if you are or not?

AMD told me this can have an impact on some games although I've not heard of any problems, and my main gaming system is an Athlon 64 X2 3800+ at the moment. Here are some additional details.

AMDZone