Well, as it is a q6600 then I would assume the motherboard requires DDR2. The common speed for such would be 800mhz, so I would say something like this is what he wants
G Skill DDR2 - $89.99http://www.newegg.com/Product/Product.aspx?Item=N82E16820231122&cm_re=g_skill_DDR_2-_-20-231-122-_-Product
However, I need to ask - what Operating System is he using? Only 64bit will make full use of 4gb RAM
How much RAM does he currently have? If he is on a 32bit Operating System then 2gb is all he really needs and will unlikely see much peformance gain from more.