Hi ethani7.
You are right, your motherboard has only SATA II ports and the SSD is SATA III. As you have already figured out, the SATA interface is backwards compatible so you should have no issues with using a SATA III drive with a SATA II port. However, as you've mentioned, the SSD's performance will be bottlenecked. You can see the specs of the SSD from the link you've posted, those speeds will be capped at 300MB/s as this is the maximum speed allowed by the older SATA II revision.
In my opinion upgrading your system with an SSD for the OS, even if it will work with SATA II is still worth it. The boost will be pretty significant and noticeable. Plus you'll get the maximum out of the SSD's faster response time.
On the other hand if...