It depends on your RAID setup.
RAID 0 will boost data transfer rate, but increase the likelihood of failure.
RAID 1 will (usually) not affect performance, but give a high reliability at the cost of double price for the same storage (as each drive must have a mirror).
RAID 1+0 or 0+1 will both boost performance and provide reliability. But again at the double price.
RAID 5 will boost reading performance and provide reliability at much lower extra cost. However, it doesn't improve (and sometimes even decreases) writing performance.
There are other setups. You can check it all at the following comprehensive RAID guide (some 80 pages, no less!):
<A HREF="http://www.storagereview.com/guide2000/ref/hdd/perf/raid/index.html" target="_new">http://www.storagereview.com/guide2000/ref/hdd/perf/raid/index.html</A>
Leo