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RAID will give a performance boost for some apps, in others you won't notice the difference. RAID = good for large sequential file transfers, bad for smaller ones, generally. Boot time, games loading large maps and network file transfers (over gbit connection) are some of the fields that will profit from RAID.
For RAID you need two identical harddrives and a compatible controller with drivers. Check your motherboard for a controller, or buy a loose PCI(-E) one.