Not all SSD's are SATA III not that it matters, if you get a SATA III SSD, you'll simply be limited to SATA II speeds, everything else will be the same.
It just supports up to up to 3Gb/s, like said above, you'll run at sata II speeds, so it wil affect the performance of the ssd, if it's a really fast one(under heavy loads), but still it's better that not having one, it won't degrade that much.