I had a friend who booted off of dual 15K drives striped in a RAID 0, hooked to a pretty hefty SCSI RAID card with gobs of cache. It was impressive, everything was practically instant. Seems there's no limit to what people going for the ultimate gaming rig will do.
I had an HP SmartArray PCI card with 512MB cache in my old desktop server booting to three 300 GB 15K Ultra320 SCSI disks in a RAID5. Server 2003 took less than 10 seconds to come up to a login screen. With 32 GB RAM and two dual-core Intel procs, it could run just about anything, but my God did it just gobble up power!