For typical desktop use, RAID is not better than individual disks; in terms of speed, reliabiltyh and/or expense.
There are two main types of RAID - striping and mirroring.
Mirroring writes whatever you save immediately to 2 disks at once. This can help you recover if one disk fails catastrophically. It does NOT help you if you accidentally delete a file, or save changes when you didn't want to, or write corrupted data as a result of a crash -- whatever data damage that is written to Disk 1 is also written to disk 2. Also, running a mirror "wastes" at least one drive's capacity, so a 2 drive mirrored RAID 1 set holds only one drive worth of data.
The second type of RAID is Striping -- where data is divided up into pieces, and part written to one drive and part written to another. This can speed up disk reading and writing, because while one disk is busy getting one block of data, the other disk can already be setting up to read/write the next block.
Two problems: It is risky: if you have a failure of either drive, ALL of your data on both drives is gone.
Second problem: Turns out that running a RAID on typical desktop machinery doing typical desktop applications, doesn't gain you much, if any, real world speed. Only intensive disk operations typical of servers, high end video capture, and very high end graphics, stresses the disk systems enouogh for the benefit of striped RAID to outweight the computation loss in calculating where the blocks of data go. Also, if you are running RAID through a single Firewire controller (which you are unless you install a PCI Firewire card), then the botttleneck of the controller nullifies part of the speed improvement.
More complex RAID systems, of 3 to 10 drives, combine the concepts of striping and mirroring in various ways. They are expensive, and again, only pay back on high-end and multi-user scenarios.
The simplest way to improve performance of a Mac';s hard drive system is just to have two drives -- one exclusively for Data, and one for your System and applications. If you have three drives, you can divide them up by System vs Applications vs Data. IF you have 4 drives, you can dedicate the fastest one to the OS scratch files and scratch files for applications like Photoshop. Obviously, there are variations depending on the expandability of the Mac, the speed and size of the drives, and the busses available. Best case on a PowerMac G5 is to install a SATA card, and have 2 SATA internal drives on the motherboard SATA controller, and 2 external SATA drives on the card.
The reason to divide Data and System is so that one set of heads can be reading/writing data, while the other drive's heads are simultaneously accessing the scratch (virtual memory) files for the OS.