What RAID level do you want to run? 0 (= striping) or 1 (= mirroring).
With RAID0, you double your capacity, and you increase your throughput considerably (the amount of data you can transfer). If you work with lots of random accesses, something like a database, or maybe a music project with a great many instruments, you won't see almost any increase in speed, because the seek time is slightly worse than that of one drive.
Obviously, copying larger files (movies or many songs) will be faster. Make sure you have a backup strategy in case you go for a RAID0.
With a RAID1, you won't see any performance gains, the second drive mirrors the data in case one drive fails.