I'm sure someone with deeper knowledge than me will step in, but from dim memory, your SATA chip will support SATA-II. However, many manufacturers limit it to SATA-I. I'd guess this would be for many reasons, including support chip specs and motherboard design.
The big win with SSD for many people isn't the absolute increase in the speed at which data is pulled off the drive, it's the absence of seek time.
Your problem is that Vista doesn't support the TRIM command, which is pretty important when getting the best speed out of an SSD. A couple of brands are very poor with Vista.
Slow boot times can be down to timeouts in badly-written USB drivers: "gee, I seem to remember this scanner from last time, let's wait 20 seconds in case it reappears".