So long as your motherboard (or expansion cards) have more slots, you can always connect multiple hard drives. But in RAID, it uses the hard drives together, providing different benefits. Generally, it will greatly improve data transfer speeds (due to accessing twice as many or more drives at a time). It also can provide redundancy against failing disks.
RAID0: Roughly double speed with double disks and triple with 3 disks. The overhead catches up so it's not worth more than 4 disks. However, if one drive fails, all data is lost. Even number of disks required.
RAID1: Faster than single disk, but the main purpose is a mirrored copy of each drive so that you have to lose the mirrored drive at the same time for data to be lost. Even number of disks required.
RAID5: Disk 0 has data A & B. Disk 1 has data B & C. Disk 2 has data A & C. If any one disk goes down, you still have data stored elsewhere. Minimum 3 disks.
RAID10/01: Combos of RAID0 and RAID 1 that use them in different orders.
I have a 3 disk RAID 0 and RAID 5 combo setup. I get 425MB/s consistent across my RAID 0 and 250MB/s to 200MB/s across my RAID 5. Install HDTach and see what speeds your disk(s) gets. Also, I'm running Samsung Spinpoint F3 1TB's, which are very fast on their own.