Do a search for "SCSI" on the BSS just in this forum and that should keep you reading for days.
To give a somewhat brief and simple answer to your questions:
SCSI stands for Small Computer Systems Interface and refers to more than just hard drives. It's a bus that can connect several different types of devices. SCSI hard drives are usually more expensive, a little more difficult to install into a PC without an existing SCSI controller, and faster for the most part. If you want more background info, use your favorite search engine and you'll get plenty of stuff. As for what you are asking concerning recording, IDE is historically thought of as slower than SCSI, and SCSI used to be recommended and used much more for intensive audio/video applications over IDE. Lately, faster IDE implementations (ATA66, ATA100, EIDE, UDMA, etc... for simplification purposes they can all be considered the same thing) have caught up enough where most people can't really justify the jump in price to the high end SCSI drives unless they just have the money and want the absolute fastest drive performance available. SCSI is still a better choice for a 10 drive, RAID5, hardware based array on a server or something like that, but beyond that IDE can do the job good enough and for less money. Some people still might swear that to do it right on a recording computer you have to use SCSI. Maybe they are right, but I think for most home recording it's just not worth the extra cost.
If you're IDE drive is 7200 RPM then you should be fine for recording. If you are looking to buy a new hard drive for recording, IDE is probably the way to go as in most cases it just requires setting a slave/master jumper and plugging in into the controller that's built onto the motherboard. 20 GB would probably be the minimum size drive I would buy now since the prices have come down so much. Find out where the current sweet spot is for price/GB and go from there. I have had good luck with Maxtor drives so far.
