taken from
http://rage3d.com/board/showthread.php?s=&threadid=33719530
Chances are most of you hardcore tweakers know about this one, but for those who havent, here ya go
Start -> Run -> REGEDIT
Browse to the Priority Control folder, which is located at
HKEY_LOCAL_MACHINE / System / CurrentControlSet / Control
In the right hand window, which displays the contents of the folder "Priority Control," Create a new DWORD value called IRQ#Priority and set the value to 1, where # is the IRQ number of your Radeon. For me it's 16, so I put IRQ16Priority with a value of 1. To look your IRQ up, open up Device Manager, double click your Radeon, click on Resources, and find the IRQ number.
Basically this allows your AGP slot a "higher priority" and gives you a FPS boost. While the amount of this boost varies from machine to machine, on mine Halo goes from 25 to 33
If you want to turn it off, just delete the DWORD you made.
Restart the machine after applying the tweak.
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I just did this, and didn't really notice any difference in FPS, but it raises it for some people. This was done with no mention to Nvidia cards, although I'm sure this tweak would work fine with them as well since it's dealing directly with the AGP slot and not the card.