As stated before, 1 HDD split into 2 partions does not decrease the seek times.
RAID0 would show decreased seek time on most applications and programs but not for the O/S.
If you don't want RAID0, get a Western Digital Raptor drive, fastest none SCSI HDD on the market at 10k rpm. Either that, or check what speed the current HDD's run at and if its less than 7.2k rpm, get a new one and you'll see a difference that way.
Also, remeber to defragment the HDD regularly so the HDD doesn't have to work as hard to find data, but thats going off the scope of this thread.
Be not afraid of growing slowly, be afraid only of standing still - Knowledge will get you somewhere, stupidity will get you nowhere