800Mhz paired with 1066mhz ram

Will you really take advantage of 6 GB of memory? If not, then don't bother. Depending on your computer, you might be able to run 6 GB at 800 MHz, 5-5-5-18 timings and 1.9V-2.0V. You probably will never get them totally stable at 1066 MHz even if you loosen the timings and increase the voltage. It sometimes is challenging to run four identical modules; running different modules just makes it more difficult.

Agreed. Just keep what you have and sell or give the others to someone who needs them. Adding 2 more gig won't make much difference, and you would likely have to run them all at 800. And, just for the record, running 800 or 1066 won't make any noticable difference either.

Well, it certainly won't hurt to put it in the machine and see for yourself. I doubt you notice any difference no matter what you do, 4gig, 6 gig, 800, 1066....try it and see what is better. People just go way overboard on RAM these days because it is so cheap. There are very few people, normal everyday users, that would ever need more than 4 gig of RAM. What game do you have that would actually come close to maxing you out at 4 gig? Most games won't even use a gig, say windows uses a gig...at most, that still leaves you with 2 gig open. While you are gaming and downloading, open your task manager and see how much memory you are using.