OK, so you've already got the Velociraptors? If it was a case of you buying them then I'd suggest spending the money on an SSD instead, but if you already have them then yes, if you don't need all the space then you can short stroke them to get a little better performance.
Short stroking should reduce the access times slightly, while using RAID 0 should increase the transfer rates. Fast access times help tasks that require a lot of small random I/O (such as booting or starting up programs). Fast transfer rates help reading and writing large files. So do whichever tweak (or both of them) that applies to the type of performance you're looking for.
Remember that RAID 0 increases the chances of loosing all the data stored on both hard drives, so be sure the data isn't critical or that you have it backed up.