That makes a lot sense now! LOL!
100X Speed is just a bit faster than a hard drive. A 200X would be the best for anything up to a G3-WallStreet, that is from a PB 520 to a G3 Wallstreet. 133X and 150X are good, but not as fast as a 200X. 400X are best for the G3 Prismo/Lombards and on, though a 200X is comfortable in those machines - all of these will be faster than hard drive.
Price and Speed is a factor.
Technically 100X is supposed be as fast as a hard drive but I find an old 75X to be equal to a hard drive; the data speed might be the same but the hard drive is doing a lot of buffering before sending and that slows it down, especially with the old hard drives. Thus 200X is a bit more than double the hard drive speed, and 400X about 4 times. Just drop the 00's.
Thing is, because many adapters wont have True IDE/DMA/UDMA support without a bit of hacking to the interface, but as is, it should be fine up to a G3 Wallstreet.
See: http://www.fccps.cz/download/adv/frr/cf.html