The theoretical limit of SATA II is 3Gbit/s or about 384MB/s. Nothing ever performs at the peak theoretical limit, so I would round that down to 300-325MB/s. The SSD you linked has a max read of 555MB/s and a max read of 515MB/s. Of course, those numbers are usually ideals as well and you'll likely never actually see performance that high. If you round down to 500MB/s for both read/write, you should see about a 40% performance drop using a SATA II controller (300MB/s divided by 500MB/s).
One of the guys I work with recently wrote an article that dealt with the actual performance difference between SATA II and III controllers that might give you some more background information. http://www.pugetsystems.com/labs/articles/SATA-Controller-Performance-Explored-87
. In this case you probably don't really care about the different brands of controllers, but the comparison of the Intel SATA II and Intel SATA III might be interesting to look at. Especially since on that SATA II controller the max read/write was closer to 250MB/s than the theoretical 384MB/s which if your controller is similar would actually be more like a 50% performance drop.
As for moving your OS, I'de recommend doing a fresh install if possible. Technically you should be able to just image over, but when you are imaging to a drive that is a different size, it's not uncommon to have weird things happen (often partition sizes will be wrong or the new drive won't boot properly)