DDR2 800 vs. DDR2 1066

I'm definately not an expert on the subject, but i'm pretty sure the 1066 pretty much just gives you more overclocking headroom; allows you to bring the motherboard up to 1066/2 = 533fsb, which I doubt the Q6600 will be reaching even with the G-0 stepping... So to answer your question, I would go with the 4GB of DDR2-800 (assuming you're using Vista 64bit to actually make use of all the memory). If you were to just OC to 3.0GHz on the Q6600, I think all you would REALLY need is DDR2-667: 667/2 = 333fsb * 9 (Q6600 multiplier) = 3GHz. And of course with the P35 motherboard you have chosen you can do this by simply changing the bus speed in BIOS w/o having to mess around with voltages

Also, I really like OCZ. It's good stuff, oh yes. Everyone will swear by the brand that has worked best for them (and the unlucky ones who received a borked stick will burn them), just look for the tightest timings/price, can't really go wrong either way.