I read around that RAID only makes a small speed improvement
This is incorrect. with the Raptors, it's 50-75% improvement, depending on the controller and set-up, not to mention the motherboard. Depending on set-up, it can be a 100% improvement on large files.
RAID-0 creates "stripes" on 2 or more drives, and uses them as one large drive. So 2 36GB Raptors become one 72GB drive. If you want more "letters", you just partition the larger volume.
What you do lose in RAID-0 is security, if one drive craps out, or the controller craps out, you lose everything. For this reason, people often do what's called RAID-0+1, where you create a second array to mirror the first. If one of them dies, the controller switches over to the other one. This is not possible with the onboard SATA controllers, as they still only have 2 ports. On a PCI controller, you have more ports for constructing such things.
You can read up on RAID someplace like the Promise site.