the core 2 extreme qx6700 has a unlocked multiplier witch means more overclocking ability higher fsb cpu speed and so on and the core 2 quad q9550 has a higher l2 cache the qx6700 has 8 mb l2 cache and the q9550 has 12 mb l2 cache i personaly like the unlocked multiplier for better overclocking but the cache realy helps to!
If your going to be using Multimedia applications , Than the Q9550 is a better choice it has the SSE Inst. up to 4.1 , the QX6700 has SSE 3.0 it's a way older CPU. Made for speed not Newer programs like Blue ray apps.
