If you're writting and transfering a large amount of data, that is a sizeable increase over what your average user and gamer would do, then a RAID-0 would be worth it. Like what was stated above, you add twice the risk of data loss. If one HDD goes, you're done. What's the alternative?
Here's a wikipedia entry on various RAID levels
RAID - Wikipedia, the free encyclopedia
You're better off getting an SSD over a RAID-0 for your desktop, unless you're doing some heavy file work.
But with your budget and wanting an i7 rig, you're better off putting that money elsewhere.