4x ATA133 drives would allow for faster sustained buffered file transfers if you put them in raid0. However, raid0 with ATA133 is not very smart unless you have a good backup system, and you are more than willing to use it.
If you put them in raid5, it would be really slow and it would eat up lots of CPU cycles, unless you bought a really expensive hardware raid card.
If you put them in raid 1+0, the read speeds would be just about as fast as in raid0, but the write speeds would be not nearly as good, however, if 1 drive goes bad, its no big deal since you don't loose any data.
4 x ata133 drives in raid0 or raid 1+0 would be the best for like, game loading times, however, for some things, like boot times, or general system swiftness, I would think a single 15K drive would perform the best.
your best bet would probably be raid0 of Raptors ... that would be much less expensive than a raid array of 15k scsi drives, and it would offer a much better seek time than that of 7200rpm ATA133 drives. It would also enable good throughput (better than a single 15K drive, though not as much as 4 7200rpm ATA133 drives).