People are hitting 3 Ghz+ with the stock heatsink. The problem is he needs to raise voltages somewhere. Vcore, NB Vcore, SB Vcore (don't raise this much, you might not even need to), and Vdimm.
Make sure he is running a 1:1 ratio with the FSB:RAM dividers. It's usually an option that says DDR2 followed by a number, I think this needs to be set to DDR2-533. It will show it increase as you raise FSB.
Hope that helps.
Some useful programs to download are Coretemp and Speedfan (temperature and fan speed monitoring) and CPU-Z, which tells you about your clock speeds and RAM timings. That reminds me, loosening up the ram timings a bit can help to keep you from hitting a wall when overclocking, and you will sacrifice little performance.
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||DFI UT P35||Q9550@3400 Mhz||Geil 4GB RAM||Radeon 5770||