A couple of notes:
SCSI the interface is barely faster than IDE except when running high workloads in some form of multiple-access app pattern. On a desktop running normal apps, it shouldn't be much faster in and of itself.
SCSI drives are generally faster than IDE, but they are smaller and they are more expensive. If they are not smaller and/or more expensive, then make sure you research it to make sure it's a good drive.
Ultra160 is way over the top if you are going for just a single drive... U2W (Ultra80) is plenty fast and the controllers are about 1/2 as much. Ultra160 really only comes in as significantly faster when you have multiple drives that are simultaneously accessing from.
Do not (I repeat: do not) just buy a cheap SCSI drive - they are not always faster, they are often hotter and can be noiser. If you want to go SCSI get something decent. Expect to pay $200+ for 18GB or more. If you pay any less you may be disappointed with the results.
My recommendations of good, relatively inexpensive SCSI drives:
18GB Maxtor Atlas II - this one can get a little hot... don't get if you have a hot case, or no space for a HD cooler.
18GB IBM Ultrastar 36LZX - this is an all-around good drive. It's cooler than most, quieter than most, but a little slower than most too. Newegg has them (out of stock currently) for $165
18GB Seagate Cheetah 36XL
Be very careful of the auctions at Ebay - a lot of the drives there are older drives, they are occassionally server pulls and may take hard-to-find adapters. Whatever you buy, make sure you buy one with the right connector for your PCI card which will probably be a 68-pin adapter.