When 3xp0 says dual 2.4 Ghz, he does mean dual processors. Dual processors can handle more commands than a single processor command. That's why the mail server where I work is a dual processor machine. It can more easily handle receiving mail, sending mail, and allowing customers access to their mail.
SCSI is a faster interface than IDE. I don't know how much faster. You will need a SCSI controller to use a SCSI hd.
I don't know a whole lot about RAID, but what I do know is that RAID will take multiple hard drives and make them look like one. It then "stripes" data between the hard drives (a little on the first, a little on the second, and so on). If one of the drives fails, it can be taken out, and a new one put in it's place, and the other drives will have enough overlap of data that they can rebuild the failed.
Personally, I'd go with RAID over SCSI, because of the last feature, but you'll also need a RAID controller.
Anything I missed?