You don't need to format and reinstall everything. If you have an Intel 865 or 875 chipset, you can use Intel's Application Accelerator Raid Edition to migrage the data from a single sata drive to your new Raid 0 setup. The migration process might take a couple hours, but it only runs at the background, so you can still surf the web and so some other stuff while it's running.
This is what Intel claims, I don't have any experience on how reliable it is. I still don't have $$$$ to do a raid 0 setup.