I get the impression that for SSD's, running a RAID setup (the one that share the data, not the one that mirrors it across both drives) doesn't produce any noticeable gains due to the way the data is written to the drives themselves.
There's supposedly a decent difference with SATA2 vs SATA3 etc with the latest generation of drives
Unless your data isn't compressible (eg movies, or whatever your working with, you'll have to figure that bit out), devices that use the 'sandforce' drivers are supposed to be pretty darn good.
Choice depends on your budget though, but if you want what's considered by most to be the "best" consumer level SSD, the OCZ vertex 3 is probably it. PCI-e plug ins are about 50% more expensive, which is crazy.
Honestly, due to the fact yuo've still got a SATA2 motherboard, i'd just pick up a OCZ vertex 2 120gb (review. They might be 6 months outdated, but are still a great drive (blow away any disc based hard drive), wont bottleneck on your setup and are cheap, $190 from umart for a 120gb or $350 for 240GB
As far as usefulness of a SSD goes, depends on what your doing. I'm not going out of my way to get one (though 200 bucks for a 120gb one is cheap) because i rarely need the speed. other people might.
As far as reliability goes, your not realistically going to run out of cycles on the memory. Read an article on that point, and they pointed out that, though there might be a 10000 cycle limit, the fact that you aren't rewriting the entire drive every time you do something it would take about 10 years of normal usage to run out.