This is what I did last night with my 7900GT :P
1. Download ATI Tool.
2. Download Rivatuner
3. Install both.
4. Overclock using Rivatuner on the core & shader only by 10mhz.
5. Open ATI Tool and scan for Artifacts for 5 minutes (keep an eye on the temperature).
6. If there are no errors then repeat step 4 & 5 until there are; then back off by 10mhz.
7. Then with the core & shader overclocked, do the same process with the memory.
8. Then you are done and have found the max stable overclock for you core, shader and memory.
Remember better cooling = better overclock and longer life of the card. Also each individual card overclocks differently so don't expect to match someone else's overclock just because they have the exact same card as you.
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