Actually there is no SATA III, and technically there is no SATA II either. The so called SATA II devices are the same generation as SATA I, with only a faster interface speed, which makes no difference unless you're RAID'ing multiple drives (more than 3).
Officially the terms are SATA150 (or SATA 1.5Gbps), and SATA300 ( or SATA 3.0Gbps). Your motherboard only supports the SATA150 interface. Therefore, if you buy a SATA300 drive (which I would.... they also have additional features, like Perpendicular recording on most Seagate Barracuda 7200.10 models), you will simply have to move a jumper on the hard drive to force it to use the slower interface. Some drives will automatically detect the fastest interface speed and use it, some need jumpers... but all SATA300 drives can be configured to run as a SATA150 drive.