There are ssd's that use SATA II and SATA III. The newer SATA III ssd's are backwards compatible with SATA II.
For example, the new Crucial RealSSD C300 CTFDDAC128MAG-1G1 2.5" 128GB SATA III MLC Internal Solid State Drive can be used with SATA II. If you take a look at the technical details over at newegg.com you'll notice that sequential access read and write data is listed for SATA II and SATA III.http://www.newegg.com/Product/Product.aspx?Item=N82E16820148348
I am still checking reviews and article to determine if there is any benefit to installing a SATA III ssd in a SATA II pc system. Temporarily, I don't think there is any benefit.