to be honest you can get away with using sata 2 for the time being and then if and when you replace your mobo etc just connect it to a sata 3 port then.
will need to change the bios options for sata ports to achi.
these sata PCI cards can say sata 3 but the speeds can be the same as sata 2 so be aware if you go down this route that it can handle the full speed of your SSD.
I have both my ssds connected to sata 2 and I can see a big improvement on the system just by using them.
if I was you get the drive and try it on sata 2, if you want the full sata 3 speeds then make sure you get a pci card that can handle the full speeds otherwise it will be a waste of money.
I have considered this myself but decided to just wait till I do a cpu\mobo\memory upgrade
is this your mobo?
http://www.asus.com/uk/Motherboards/P6TD_Deluxe/specifications/
as it states that the mobo has 6 x 3 sata 3 ports on it so you would connect the ssd to one of them