You also need to decide what kind of RAID you want... RAID is a generic term describing multiple-disk setups ("Redundant Array of Independent Devices") that somehow interact. There two most widely-used are mirroring and striping.
If you use two 120GB drives in a mirrored array, then you'll have a maximum RAID capacity of 120GB, since one drive is just a "backup" or mirror of the other. This is good for data integrity, since if one drive fails, you can replace it and rebuild the RAID from the other drive without data loss.
If you use two 120GB drives in a striped array, then you'll have a maximum RAID capacity of 240GB. Striping a disk set writes alternating bits of data, or "stripes" it across both drives. Good for increased speed and decreased latency. Bad thing is, if one drive fails, bye-bye data!
These two are available with Mac OS X's Disk Utility. There are other RAID setups, like striping with parity for error correction/detection and so on. Might wanna do a Google search and research what kind of RAID array you want and what would suit your usage requirements.
If you wanna just "play around" with the whole RAID thing, I would get two identically sized drives (preferrably the same exact drives) and a PCI ATA RAID card. PCI RAID cards do the whole RAID thing in their hardware, whereas Disk Utility does it in software -- and we all know hardware is faster than software.
I would also suggest keeping your boot drive on a separate, non-RAID volume. I doubt you'd see much speed improvement on a desktop workstation like yours simply by putting the OS on a RAID set as opposed to getting a fast non-RAID PCI ATA card and putting your boot drive on that.