SATA II versus SATA III - Both SSD's and HD's can b SATA II or SATA III. This is the speed of the "interface between the device and the rest of the system, not the speed of the disk itself. When ya hear peeps say that a SATA III HD won't perform any differently than an equivalent SATA II unit, it's not quite true. The mechanical drive may not be able to transfer at SATA III speeds but the drives cache can.
HD's versus SSD's - SSD's have higher transfer rates than mechanical HD's. The 1st time ya boot ya system after adding an SSD, you will notice a significant boot up tome increase .... after that, it gets somewhat hard to notice any significant difference outside special doisk intensive activities.
If ya want ya programs / games to gain the advantage, you're gonna need room to put them on that SSD. Search these boards and you'll see more than 1 person dispappointed with their 30, 40 or 60 GB SSD's in this respect. You can cut this don by moving your user files off the C and onto the HD, make sure to put mail there too.
But my suggestion would be a Vertex 2 of 120 GB size....curently under $200 on newegg