I'm assuming you mean RAID 0, which increases performance.
<A HREF="http://www.techweb.com/encyclopedia/defineterm?term=raid" target="_new">Here</A> is a good explanation of RAID.
Now, as to your original question: yes, it will increase performance (in a way). It increases how fast information is read from the hard drives (or written, for that matter), so the game textures will load into the RAM faster. You won't have higher framrates, but Windows will load faster, games and programs will load faster, and less waiting time between levels (unless you're waiting on a server or something).
Is it worth the cost? I apparently thought so, since I have RAID in my computer. But you'd see more difference in games if you saved the money and went with a good video card. Both would of course be the best.
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