I had the same dilema myself a while back. I didn't bither with the card or a mobo upgrade, as the guys here suggested. The SSD is on a SATA2 port and works just fine. Its performance is far better than the mechanical drive I used to have and I quickly forgot about installing the SATA3 card or upgrading mobo.
If you have or plan on getting an SSD you could always install on the SATA2 port and see how well it runs. If you're happy with it all is well, if not, try a card later.