Cost per performance wise most people would say avoid 15k hard drives alltogether. You can get similar if not better performance with high capacity SATA II 7200 RPM and even better with 2 of them using a raid mirror configuration plus it will be pretty quiet as is.
If you really do want to go with an SAS or raptor configuration i would be interested to know of your results but to quieten it down there are a few solutions but the one im looking at for my HTPC is
this. My friend (nightshadow) uses one with his raptor drive and says it works wonders!
Quiet PC are doing a series of shows on Hexus TV about quieting down your computer and checking their website out i found quite a few other solutions.
Check them out!