yeah, raptors are no doubt less cost effective per gigabyte... at the very least, it would make a for a responsive system when used as an OS & app hdd...
for example, the cheapest current raptor is $100, for the 36GB 16MB ADFD model (its the same speed as the 74GB ($150) and 150GB ($200+) ADFDs, mainly due to them all having the same sized 74GB platters)
but, that will certainly make your system more responsive (and offer most likely better performance for most desktop situations) than going with raid 0, for *possibly* less money
then just have a larger slower hdd for storage, media, and anything else