Yeah, you're probably going to choose between two types of RAID - 0 or 1. There's more, but you probably won't use them.
1 is Mirrored. I don't think that's what you're thinking of. It actually decreases or keeps the same performance because it basically makes identical copies of the hard drives. So when you write a file, it goes on both hard drives. This is for reliability, if one fails, then you still have all your data.
0 is Striped. A part of the data goes on each drive. So access times are decreased since each drive is accessing part of the file, as opposed to a single drive just reading the file. But on the downside, if one drive fails, you lose all the data.
C2D E6600 | 4GB DDR2-800 | 9800GTX+ | Asus P5B-E | 150GB Raptor | 320GB 7200.10 | 750W Xigmatek PSU