I'm currently running windows7 x64 bit. I did some research and heard people say that the more slots are filled, the more strain it puts on your memory controller and the harder it will be to overclock. Also, right now the 4x4GB kits are cheaper than the 2x8GB kits, but the 2x8GB kits have a higher CAS latency compared to the 4x4GB kits. So I'm kind of in a debate right now.
windows7 x64 bit what? Home or Pro....what version?
Nothing u will be ever to notice, that ...and the harder it will be to overclock.
How far are u going to OC? Beyond 5GHz, maybe it will be to overclock there, but less than that, no problem.