I'm about to buy a new PC (not totally decided yet, it will be something like: Q6600, 2/4GB RAM, 4870(X2)), I want to OC it (that's why I got this CPU) and read about synchronous mode - where the FSB and memory speed are the same. Does it help?and if I want to get to 3GHz (9*333) I need to get 667 memory? or should i get 800?
Pretty much the only memory that you should buy these days is DDR2-800. DDR2-667 isn't really cheaper any more, and DDR2-1066 is only needed for monster overclocks. Most DDR2-800 is good for ~900MHz/450MHz FSB, most people don't go higher then this. Taking one of Intels 266/333 chips and going to 400 is mostly what people do.