Been there, done that (2x15k RAID0), found they were totally unoptimised for desktop usage patterns. Just one of the many stupid things I did during my teenage year.
The one and only 10k drive I found that is optimised for desktop usage is VelociRaptor. The previous two generations of Raptor had hardware & firmware derived from WD's SCSI line instead.
Four Savvio 15ks in RAID0 might hold up against a single Indilinx/Intel SSD with up to three simultaneous disk usage. Once you go beyond that then any 15k drives will crumple to their knee. That's the weakness of HDD and the potential to be exploited by SSD users.
On the $/GB side of things, it is possible to find OCZ Agility 60GB (Indilinx) at close to $2.1/GB via sites like Slickdeals occasionally. IMO $2/GB is when the value of SSD becomes significantly greater than VelocityRaptor for desktop usage (when harshly utilised).
Most are still currently between $2.8-$3/GB.