I have to update a bit. It seems that we will get faster SSD's sooner:
http://hothardware.com/News/Microns-RealSSD-C300-SSD-Is-The-Fastest-Ever/
They will be pricey, but will need a 6gb sata to run at full speed.
If I were to buy a new motherboard today, it would include 6gb sata and usb 3.0.
As to raid-0 for the OS, don't bother. It will work, but there will, on balance be no performance impact.
Raid-0 distributes alternating stripes of data on each drive. The OS mostly does random reads and writes which will be satisfied by one drive or the other. Only if you were doing sequential operations of large files is raid-0 of much help. Then, the reads could be done simultaneously.
If you want the best performing OS, get a good SSD with trim support. A 40gb drive goes for $100-$150. Pair that with a 1tb storage drive and you will have a good system for today.