It's probably not the PSU (yet). I'd say the rams can't keep up with the cpu speed. There's a lot of (simple) math involved when overclocking, and overclocking results can often be predicted by comparing them with "threshold values" that other overclockers have reached. But since it would take too long to explain, here's the gist of it so you know what to look for:
1. reduce your HyperTransport (HT) multiplier to 4x or 3x
2. loosen your ram timings (you might have read "4-4-4-12" or "CAS4" and other similar numbers)
3. put a ddr2 divider (pretty much means change the dram mode to something lower, like ddr2-667 or ddr2-533)
4. raise dimm voltage
5. raise cpu voltage
The voltage must be raised to increase stability, otherwise it will fail to boot or programs may crash at runtime. Raising voltages will raise temperatures as well so don't overdo it. Corsair should have mentioned your ram's specifications such as maximum voltage somewhere. You won't be stable at 3.0GHz without additional cpu voltage, that's for sure (X2 6000+ is around 1.4v or 1.45v iirc).
Try overclock.net or some other overclocking site. You can check out other people's settings and get a feel for what it is you might have to change. There are also better explanations of the math behind overclocking as well.
Also, higher cpu speed isn't always better. you have to keep an eye on your HT and dram frequencies and make sure they don't drop too low. I suggest you make sure your dram frequency is near their rated speeds, while HT frequency should be near 1000MHz at least.
And your overclock isn't stable unless you can run stress tests without crashing. Game benchmarks won't cut it, you need to use software like OCCT (which is free) or Everest (which isn't) and run the test for a few hours before declaring it stable.
Just so you have an idea of the possibilities, my X2 4000+ is at 2.9GHz with only the stock cooler. Temps are high already so I'll push further in the future when I get a better cooler. My PSU is only 416W, my video card is a X1650 XT and I have 3 hard drives.
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