Overclocking 8500

I just got watercooling so I wanted to overclock my vid card. I have rage3d and I can't go over 315 gpu clock because I get the vpu recovery error, I don't get any artifacts. If I open cs I get the error but can go back into the game. I don't know a lot about overclocking but do I need to raise the voltage or something? Or is this just the limit of the card? I've tried searching but I don't know enough about it to find good info. Thanks guys.