superboyac: in my opinion you only need one raptor drive (unless you're going to set up a RAID MIRROR for data safety (and remember that mirror is a supplement to, not a substitute for, backups)). If you go for a 150gb velociraptor, you have more than enough space for OS + programs + documents, and you can use whatever other drive for bulk data storage. Other drives are plenty fast as well these days, and some of the larger drives beat the original raptors in term of sustained data transfer rate (although not seek time).
With a decent case that reduces vibrations, the 10k rpm of the raptors is not a problem at all. The only time they make some noise is when the read/write heads move a lot, which should only happen on badly fragmented partitions, or when you do multiple I/O streams on a single drive. And even then, it's a relatively pleasant click/clack noise that, although slightly loud, isn't annoying like the high-pitched rotation whine you can usually hear from drives (but which I, strangely enough, don't hear from the raptors... perhaps the 10k rpm noise is too high-pitched? Or my case dampened enough?).