depends on your budget, i reccomended 8Gb for some1 because they were having some great Quad extremem processor and had a really big budget,
if your gonna get a Q6600 4GB will be perfect but IMO
if your gonna a Quad extreme get 8GB if youve got the budget because it wont bottlekneck at all, where as 4GB would be very slightly bottleknecking a quad extreme,
but the general Q6600 4GB is perfect