What to do now

So unless your on a tight budget, Why not go with a latest gen card such as a Vertex3 and then If/When you upgrade, you have a better part. I"m sure someone will post about SATAII vs SATAIII Specs and if it can saturate the SATAII Controller. I'm not current with specs, theoretical performance vs actual and then if a SATAII could even be the bottleneck.

Look at it this way. If a Vertex3 can max out your SATA2 controller, Your getting the best you possibly can with your motherboard and then you could get a little more after your next upgrade. If you get a Vertex2, your stuck at those speeds after you upgrade. I say get the better SSD, upgrade motherboard when you can and get the most out of what you currently have.

It won't matter if you use a SATA 2 or 3 drive, either will work with your motherboard. SATA 2 vs 3, don't think you will run into any issues using the max bandwidth unless you start to use RAID0 or 5 with a few SSD disks together. Even then, the performance is more of a benchmark thing, you won't be able to tell much if any difference just by how fast the PC feels to you. It takes about a %30 difference in speeds for a person to notice how much faster something is running, SATA 2 vs 3 will not get you that.