I recently bought 4GB of this RAM: OCZ Vista Performance Gold Edition DIMM Kit 4GB PC2-6400U CL5-5-5-18 (DDR2-800) (OCZ2G8004GK).
I just switched it with my old PC-5300 RAM and it worked fine, but only at 333MHz like the old RAM. But now I read that there would be a large performance increase if the memory would run at 400MHz as it's intended to. And here's the problem: I have no idea how to do this.
I've read several instructions on this, but I don't dare to do it. I need my computer for university every day, if I destroy it that would be a huge problem.
So I would really appreciate it if someone could give me a step-by-step explanation of which settings to change in the BIOS.
Please note that it generally shows that increasing speed without tighter or at least the same timings does not give the same boost as getting higher speed and tight timings. @ 400 (800) you will have to run loose timings of 6,6,6,18. That it self may make it even less of a boost over your current 667 5,5,5,15. Your best bet is to check with apps you use, but i think you will not see much difference.
If it helps any I have a P5W system(Same cpu as you) running 266 3,3,3,10(i think, i do not even remember it was long ago and i am too lazy to go check) with 400 rated memory
If its stable and fast enough for you, i would just leave it.