DDR2 RAM, 1066 or 800?

For a ASUS P5Q-E motherboard and E8400. I am thinking a 2x1 kit, with another kit some months later.

I heard of problems running all banks with 1066 on some motherboards.

Also can I keep dual channel when using 2x1 and 2x2 kits on the same motherboard for a total of 6 GB with a 64 bit OS?

Yes you can run dual-channel with 2x1GB and another 2x2GB config. I don't however see a point in that.

Most mobos cannot provide enough power to all 4 DIMMs, especially for a 1066mhz spec RAM where mem voltage could be anywhere from 2.0v to 2.2v depending on the ICs. While you can basically underclock the RAM itself and reduce mem voltage, it is in another way a waste of money getting 1066mhz RAM. More over you'll need to sacrifice rather lax timings of 5-5-5-15(or in some cases even higher).

The best deal though is that I suggest you get a 2x2GB DDR2-800mhz right now. IF,in a few months time you see the need of needing more than 4GB of RAM(which is rather unlikely at this point in time, then get another kit of 2x2GB.

2x2GB 800mhz is sometime 50$ in sales good Quality
common price is 75-99$ from ncix.com country flag on the left usa / tigerdirect.com / newegg

for your ram Question this benchmark will tell
http://www.bit-tech.net/hardware/2008/07/08/is-more-memory-better/1 for Quantitys Benchmarks
http://www.anandtech.com/memory/showdoc.aspx?i=3053&p=7 Ram SPeed Benchmarks id like to see new one tough