In the options you can unlock voltage control. Providing you have the same revision of card I do (the first one - ATI reference design), you should be able to up the voltage a bit and get more out of the GPU at least. I couldn't up my memory without it artifacting so 1300MHz is what I'm getting out of it. As for the GPU however, I've got it at 1000MHz with 1.200V (default voltage is 1.162V).
As with all these things, make sure the temperatures are ok. Generally try to keep everything below 85Â°C and you're fine. Take special notice of the VRM temperatures as these tend to rise quite a bit with voltage increments, which you'll probably need to stabilise more than 900MHz or so. Use GPU-Z to check them:
Nothing seems to work on it so don't worry. The only thing that works is what you want, "Test for Stability". Click this button and watch your temperatures as this really stresses the graphics card!
If you can run it for a few minutes (don't go overboard, this isn't a CPU overclock where you stress test for three to twelve hours!), you're good. You can then test it all out in games. As a guideline, as long as your temperatures are ok, try to keep your voltage to a maximum of 1.350V. Anything below that is fine so you can probably get more out of the GPU than 1000MHz.
That should be fine as it does the same thing really. There's also Furmark although I haven't used that. OCCT also has a graphics card stress test. The good thing about GPUTool is that it looks out for instabilities and actually tells you when errors occur (ATITool does the same I believe, having used it in the past with my NVIDIA GeForce 8800GTS 640MB card).
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