The 100Mbs refers to the <i>interface </i>that the drives can connect to. Whether you have 1 drive or 4, they can connect to an interface with a capacity of 100Mbs. No drive on the market can sustain even half of that on its own, but use RAID and you might be able to get close with 4 of them (you lose some efficiency as you group them together, so 4 drives that can do 30Mbs will not give you 120Mbs even if the interface could handle it).
Have a look on Tom's site under the Storage area, and look back to when the IDE Raid cards came out (Promise did the first ones). Tom did some reviews on what you can expect from a set of IDE drives running RAID.
On a separate note, the drive speed does mean better performance, but only for the same spec of drive. An older 10Gb drive running 7200 will be beaten by a newer 60Gb 5400 drive, 'coz the data is packed closer on the platters, which means the head doesn't have to travel so far to read the same amount of info.
Check out the WD new 5400 drives that have 30Gb platters (WD300AB & WD600AB). These are very close to the best 7200 drives for performance, and are cooler and quieter in running (due to the slower spin speed). The performance is made up by packing the data in tighter. For reference, the fastest are the IBM deskstars which are 15Gb and 20Gb per platter depending on model.