You can have several drives!!! I have 3 in my box, one is using a SATA PCI card adapter. That sucker is fast, the other 2 are IDE. I think I have 440gb total internal and a 220gb external.
You can always install a drive and transfer files over to the new one. I try to dedicate a drive for programs and others for docs. I dont know if its a really good idea or not, but it makes sense in my head.
