Ok look, the people with a Q6600 if you want to overclock it to 3.6GHz or below then go for the 2X 2GB Memory Kits (6400 rated).
If you want to run your Quad higher than 3.6GHz you will either have to get the 2x 2GB Mushkin kits 6400 (vastly expensive) or go for 2X 2GB Memory (8500 rated). You could also go for 4x 1GB Memory like the Crucial Ballistix 5300 kit, though i would only recomend them if you have one of the newer P35 Chipset boards since the memory controller will handle 4X 1GB Memory modules with little problems, unlike the older chipset boards.
If you want to run your Quad higher than 3.6GHz you will either have to get the 2x 2GB Mushkin kits 6400 (vastly expensive) or go for 2X 2GB Memory (8500 rated). You could also go for 4x 1GB Memory like the Crucial Ballistix 5300 kit, though i would only recomend them if you have one of the newer P35 Chipset boards since the memory controller will handle 4X 1GB Memory modules with little problems, unlike the older chipset boards.