Yes, speed matters. I have 2 pc's at home. My main rig & my family pc. Obviously my rig is WAY faster, but it lags behind in terms of loading Vista from power-on. My OS is on a oldie 80 gig sata whereas the other os is on a SATAII partitioned in half. However, after loading programs, my pc owns.
The reason I do this is for maintenance - daily backup of OS drive to storage drive. In the event of a screwup, restoring to yesterday takes 20 mins. Also, I can run a full chkdsk on C while watching videos/listening to music on D.