Raid controllers will benefit more than anything. The current PCI standard hit's a limit of around 120Mb/s, that's not enough to handle two raptors in a raid 0 array, at maximum transfer rate.
Let alone the 4, 8 and 12 port sata raid cards (and SCSI). But this is, as usual, for those of us that like more power than is really necessary.
Intel Core2Duo 6400 @ 3.4Ghz
4GB Geil 6400 Ultra
3 x 500 Gb Samsung HD
EVGA 8800 Ultra OC