I have a Phenom X4 9950 and thinking of getting an Asus M3N78 Pro with Corsair Twin2X 4096-8500C5D Dominator EPP.
I know, unfortunately that MoBo isn't good at all for OC, but seems to be the best one for HD playback.
1. I've read that with that Mobo:
there is no control over memory timings
"This can seriously hurt your performance if you have memory that operates better than JEDEC specs"
memory timings are read from SPD (Serial Present Detect) chip on memory modules.
First, you are a little overconcerned about memory performance.
You won't be able to tell the difference for what you are going to be doing between 800, 1066mhz, or even the timings difference, but we all want that inside assurance that everything is running at it's very best possible, right? Okay.
You should indeed be able to get the timings at 1066 stated, since they are tested at this speed and rated at this speed.
Your board will likely detect them at 800, and probably set the voltage at 1.8.
Install 1 stick first, go into the BIOS, and reset the timings, the speed and the voltage to the correct specs for 1066.
The Phenoms by default will run the memory unganged, not in dual channel, if the BIOS is recent and has support for them.
If you have read up on this, it is because the Phenoms have 2 memory controllers, and if you run the memory in dual channel mode, each controller has to wait on the other to finish, non-dual channel mode means each controller will work independantly from the other, and reduce latencies. Some boards will let you change this if you want, some will not. Of course whether it helps or hurts depends widely on the application, but overall performance is slightly better running them unganged, or so I have read on a couple of sites where they ran test benchmarks. Don't remember where they were, you can probably Google and find some info.