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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Mobo: DFI LP X48 LT T2R | Bios 12/24 | XSPC Delta Chipset
CPU: Intel Q9550 E0 @ 3.6ghz (424 x 8.5) | Swiftech GTZ
Ram: OCZ PC2-9200 4GB (2 x 2G

Flex II @ 1121mhz 5-5-5-15 2T
GC: 2 x Sapphire 4870 1GB @ 780/4000
SC: Auzentech Forte 7.1
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OS: Windows 7 Home Premium 64 Bit