For RAID I would recommend using a minimum of four hard drives, or you'd be better off with them as separate disks.
For proper RAID support you should look for a card based on the PCI express bus (x16 ideally, but x4 or x1 if you don't have any spare) because the bandwidth limitations of PCI are too low to support several drives at a decent speed. You also ideally need a hardware RAID solution, as this will allow a contiguous array even if you have to reinstall your OS, and will provide far better performance as your CPU doesn't have to process the data.
You should realistically only be looking at RAID1, RAID0+1/1+0/10, RAID5, RAID6 or RAID50. RAID0 is not something I would ever consider due to its poor redundancy.
Examples:
Individual drives: 2x 500GB drives -> transfer speed low, total space 1TB, no redundancy
RAID1: 2x 500GB drives -> total space 500GB, transfer speed low, single drive redundancy
RAID5: 4x 500GB drives -> total space 1500GB, transfer speed moderate,
single drive redundancy
RAID6: 4x 500GB drives -> total space 1TB, transfer speed moderate, double drive redundancy
RAID10: 4x 500GB drives -> total space 1TB, transfer speed high, double drive redundancy
RAID50: 8x 500GB drives -> total space 3TB, transfer speed high, double drive redundancy
