You could get a single WD 750 GB disk (WD7500AAKS) because it's faster than the 320 GB and 500 GB versions and cheaper than the two together. Make a 100GB partition for the OS to keep things clean.
GA-P35-DS3L does support SATA2. SATA2 and SATA are the same thing, i.e. the latest, 3 Gbps, version of SATA. There's also SATA 1 which had half the bandwidth, but that's not in use any more except for a few older disks and motherboards.