The difference is in the actual connection to your motherboard. An ultra ATA ide drive still uses the old style ribbon cable. SATA is the newest thing and the direction drives are going. These drives are faster, and you will notice a difference if you actually use your machine for heavier performance things such as gaming.
I, personally, run 2 80 Gig Serial drives in RAID-0 on my tower with several other drives in the same machine still. This set up is extremely fast (faster than a single 10,000RPM Raptor drive), but also is not as reliable. You need to keep data backed up elsewhere because if one of your drives fails, you are dead and need to build it from scratch again.