Just a tidbit on FireWire:
FireWire is Mac trademark, i.Link is a Sony trademark but both refer to a standard link known as IEEE1394.
Anyhow, 1394 is more comparable to USB than SCSI or IDE.
1394 is sort of to USB what SCSI is to IDE, 1394 is a higher bandwidth and much more effecient protocol, but it's used for very different things, and less commonly.
It's an external connection type that's hot pluggable. Sony uses it for Digital cameras, there are external hard drives on it, Yamaha makes CDRWs on it, but generally for your main hard drive you probably want SCSI or IDE. 1394 hard drives are good for portability but they aren't really meant for a main hard drive yet (AFAIK they aren't bootable..) and they are a fair bit more expensive than IDE drives, but won't perform as well as SCSI. So if you want super performance you want SCSI, cost effectiveness is IDE. You don't want your main hard drive on 1394.
However if you are looking for a fast portable storage and you have firewire on all the computers you want to use it for then a firewire hard drive might be a good choice.
Keep in mind 1394 PCI cards are still a bit pricey and most (PC) mobos don't have 1394 built in yet.
<<What is the main advantage of firewire over SCSI and IDE?>>
Portability over IDE and internal SCSI, easy of use and hot swapability over external SCSI (of course SCSI can be configured for hot swap but it's expensive).