The various SATA versions have been designed to interoperate with each other, so you should have no problem plugging a SATA3 drive into a SATA2 port. And no hard drive is actually fast enough to saturate a SATA2 connection, so doing this won't hurt your performance. It's only some of the very fastest SSDs that really need a SATA3 port for peak performance.
Very occasionally a particular drive and a particular motherboard controller won't play nicely together, so most drives have a jumper that lets you force them to the slower SATA speed. In the very unlikely event that you run into this issue you can just use the jumper to make the them work properly.