I don't think you'll see any difference. I have Windows 7 64-bit, and I had 2x2GB DDR3 1600MHz sticks (the core i5 quad-core cpu runs them at 1333MHz). I recently added 2 more 2GB sticks, total of 8GB, all slots filled. I didn't notice any performance difference. The only reason I did that is that I like messing around with virtual machines, which need a lot of RAM. For most users, it doesn't matter. I think the only downside from filling all 4 slots might be with overclocking, not sure.