I've had a RAID0 raptor setup and trust me, SSD's blow it away.
Get a 30GB Vertex or 64GB cheap Samsung. Both around the same price as a 150GB velociraptor, and much faster, on top of being silent. When the PB22-J's are in stock they will be an even better choice giving you both great sequential speed and size.
Mechanical drives are good at one thing only - sequential file operations. As soon as any multitasking happens, or you are doing things like extracting or installing from a file on the same drive you'll be lucky to see a tenth of the rated speed.
With an SSD it doesn't matter anywhere near as much how you are using the drive, on the SSD setup I just built for a friend I can go from the start of the windows load screen after POST to using firefox in ten seconds, there's no waiting around for all the various taskbar apps and services to load becasue the ssd handles multitasking well enough to load them all at once.
Same sort of thing with opening a folder with hundreds of files, on an ssd there's no delay whilst the OS looks up the details and loads the thumbnails.
All the little things add up and you find general usage of your computer a lot more pleasant, which is why an SSD will be my next upgrade despite using a two year old 8800GTX.
Any time you hear your hard drive crunching is a situation that would be improved by SSD's.