since I was a noob once, and people ignored my posts at the beginning as well, I will try to answer you 🙂
Scsi hard drives come in size denominations of 9 gig, 18 gig, 36 gig, etc. Generally you need a controller card that fits in a pci slot, attaching cable ( including a device called a terminator) and a scsi drive.
The two main manufacturers of Scsi adapter cards are Adaptec, and Tekram. Both work well. When purchasing a card for the first time , be sure to buy a retail card" it'll include all the cables and terminator.
There are a bunch of drive manufacturers, so it's a combo of personal taste, track record, and how much you want to spend.
Then there's the speed factor. With speed comes heat. 7K, 10K, and now 15K rpm drives are available. I have been running 10K IBM drives for about a year and a half: plenty fast for me.
Card is installed in the board with nothing hooked up to it. Make sure it's recognized by Windows. Install drivers for the board. Shut down, hook up HD and cables. Set jumpers on back of drive. Reboot, make sure scsi card sees the new hard drive. Run fdisk to make partitions and format- you're good to go 🙂
Resinboy