ATA 100 has a potential burst transfer rate of 100Mb/sec.
The best benchmark I've seen gives it sustained DTR of around 30Mb/sec.
You can buy an ATA 66/100 card from Promise or ABIT (probably others as well, but these are the best known). These manufacturers also make RAID compatible cards, such as ABIT's HotRod.
The Cheetah X15 is a 15,000 RPM drive, with a potential transfer rate of 160Mb/sec (Fast U/W SCSI). SCSI RAID is the fastest disk technology commonly available (apart from solid-state disks, of course!).
The fastest part, of course, is the access time. IDE is doing well to achieve 12m/s (measured, as opposed to manufacturers' figures). Fast SCSI achieves nearly half that - hence faster sustained rates, and on a caching RAID array with, say, 128Mb cache, it's even quicker.
Access time is a simple equation; if a disk is spending longer finding the data, it's not going to transfer it as quickly. Therefore the lower this figure, the better. Never mind 7,200 RPM or whatever, it matters more how quickly the disk can find the data than how quickly it spins - although this is a contributing factor.
Drawback - SCSI costs more per Mb. Simple theory - the faster the storage medium, the more expensive it is per Mb.
IDE < SCSI < RAM < Cache
I hope this information is useful (and interesting!)