If you get a decent SSD, you don't need to have them in Raid. A single drive will work just dandy.

I'm running an OCZ SSD in conjunction with a 7200 RPM HD. I load three WoWs from the SSD, while the other two load from the HD.

The difference is remarkable because the SSD WoWs load up so much faster than the other two windows. If I run into Dalaran, the SSD-based characters follow, while the others will lose follow.

You can run the same test if you get an SSD. Keep one WoW folder on your HD and pop the other one on the SSD.