RyanW2050,
Whatever connections you have available on your motherboard or PCI card will determine what type of drive options you have available to you. Most boards going back as far as Athlon (non-XP) will support ATA100. These are the 80-pin cables that can take a huge amount of space unless you fold them neatly or use/make rounded. Serial ATA 150 and Serial (SATA) 3.0GB/s drives can only be used if your motherboard or PCI controller card supports them. They have very thin and thus better cables and tiny headers that are often colored bright colors.
The SATA drives (either flavor) support higher transfer rates than ATA100 and ATA133. The interface is rated at 1.5GB/s and 3.0GB/s but your drive will not be able to come close to that in real world performance. SATA drives can support NCQ or Native Command Queuing. NCQ allows software built into the drive to reorder requests so that data that is currently under the read head is read prior to data requiring a "seek". This improves performance especially if you are using multiple applications simultaneously. As you start to run more and more applications simultaneously your system's requests to storage start to resemble those of a server so NCQ can help if that is how you typically work with your system.
Cache is king. The more the better. I would not recommend purchasing any drive with less than 8MB. It does make a huge difference.