ABT says that Microsoft has issued a fix for those using ATI HD4870 X2 and HD3870 X2 cards that are getting poor performance.

The problem appears to with with Active State Power Management and all the patch seems to do is disable it for the device alone. If you are having issues with your performance, give the patch a shot.

According to Microsoft this issue happens according to the following scenario:

You have a computer that is running Windows Server 2008 or Windows Vista Service Pack 1.
The computer is equipped with Peripheral Component Interconnect (PCI) Express Interface slots.
Active State Power Management (ASPM) is enabled in the computer's BIOS setup.
The computer is installed with ATI HD3870×2, ATI HD4870×2 hardware, or other hardware that has a PLX Technology Inc. PEX 8647-AB PCI Express switch.
The system may show low graphics performance, and occasionally the system or certain applications may respond slowly or may not respond at all.