Lets say that i get some good ol' ddr2-800 G.Skill CAS 5 RAM.
Can i underclock it to 533mhz (on a core 2 duo), and lower the CAS to 2?
Wouldn't that offer a performance increase as long as I don't overclock?
533mhz vs. 800mhz on a stock setup is an equal contest right?
"[O]n a core 2 duo" is irrelevant, you want to be looking at the motherboard, or more specifically, its chipset. To answer your question about "533 vs 800", the answer is simply no.
When you start up your computer activate the BIOS by hitting F1, F2, F11 the delete key, or whatever the screen says to activate the BIOS. If you're not familair with it, it'll look really blocky with everything.
For my own motherboard I had to go into Chipset something to muck with the memory. For myself I could set the speed of my memory to 400, 533, or 667 speed, and 3, 4, or 5 for CAS latency. I'm assuming that CAS 3 is the lowest supported speed for DDR2 RAM, but I could be mistaken.
Past tests that I've read about say that for every full stepping of RAM speed (400 -> 533 -> 667 ->800) with a corresponding increase of CAS latency (3 -> 4 -> 5-> 6) shows an improvement of approximately 4% overall.
Underclocking, unless for cooling purposes, will be almost pointless. My opinion anyway.
Disclaimer: I know almost nothing about this kind of stuff. Someone more knowledgeable then me should post and either back-up or refute the above. For the original poster's safety.
Edit: I type too slow, follow the advice above.