Most motherboards have onboard RAID support, so I wouldn't worry about a card.
The level of RAID you pick depends on how many drives you have. RAID 0 and 1 require two drives, RAID 5 needs three, and RAID 10 needs four. RAID 0 uses data striping to arrange the contents of the disks to allow for faster access. RAID 1 implements parity, so that one drive is a clone of the other (not a true backup and should not be relied on as such). RAID 5 and 10 are a mix between the two. This is an overly simplified description, of course.
Some things to think about - the drives should be identical. Don't try to hook up your HDD with the SSD. Buy several of the same drive in one order. Also, a normal hard drive will often not work. For example, if you try to use a Western Digital Caviar drive (Green, Blue, or Black), it will continually drop out of the array due to a timing discrepancy. You'll want to buy a special "RAID Edition" drive (marked RE4) if you go with Western Digital.
Is RAID worth it? It's complicated, and if an array with striping fails, it can be almost impossible to reclaim the data. You might be better off with SSDs, since the speed increase they allow blows RAID out of the water.
I use RAID 5 with three 500GB WD drives, by the way. It's caused me a few problems.
