Alot have been said about RAID0. If the proper conditions, RAID0 can increase sequential throughput (MB/s) and I/O operations per second (IOps) when doing random-like I/O. I would say the IOps are the most important. But there's latency too, which becomes especially important with things like application-loading and booting, but also general system responsiveness.
RAID0 cannot lower the latency of your mechanical drives, but it can increase performance of desktop and server applications. Its a misconception that RAID0 only increases performance when handling "large files", the IOps story is more complicated and requires proper setup of your RAID.
For one, you need alignment. Windows XP and below align partitions improperly. So you need at least Vista/Win7, in Linux this is pretty straightforward too.
Then, you need a large enough stripesize, 128KiB or larger, to allow one I/O request to *only* be handled by one disk in the array, and not overlapping data on multiple disks. The idea behind this is that each disk can process a different I/O request at the same time, theoretically providing 100% scaling with the number of disks. But reads/writes may not be spread evenly to all disks, so this number is lower in reality when doing random I/O.
Story short, it can provide some additional performance if properly setup. However, given that SSDs can be thousands of times faster than HDDs when doing "difficult" random-like I/O, a good SSD could outperform many Velociraptors in RAID0.
That said, velociraptors do pack a reasonable 300GB of storage and are among the fastest of desktop-class HDDs.