"Can I install a Sata drive...remove one of them" - The simple answer is yes you can, without removing one of them (unless your case has no room, but this can be overcome). You will need SATA drivers click here.
"Also what is 'Raid'?" - click here.
"Can I install more than two Sata drives?" - Yes, you can, if you get a SATA PCI card click here. G