Have a look at PCI-Express 939pin motherboards, eg the Asus A8N-ASI. They're more expensive, but they come with a lot onboard, and you can drop a 6600GT or two in there. 939pin motherboards will support _any_ 939pin CPU, you don't have to worry about that AFAIK. 940pin are server based solutions, and are pretty expensive. 754pin are losing support.
edit : You might as well get 2*512, because it's cheaper, and there should be at least one or two slots left for further RAM upgrades. SATA is supported in all 939pin motherboards I've seen, that should not be a problem.