300gig and 200 gig will stripe into a raid 0 of 400gig in size. You will have 100gig left over on the other drive. However this is not always supported by your raid controller. All the hardware raid controlers like LSI and Adaptec I know will do this. Driver based raid controlers that most people have built into their motherboards are very hit and miss if they will raid unlike size and speed drives or let you access unused space on a larger drive.

As for performance, You will see an increase performance with read and writes. But you need to assess if thats where your bottleneck truely is. For WoW. Its universaly Ram then CPU then GPU before hard drives become a real factor with wow.