WD Caviar Black Raid 0 vs. SSD

I have the Western Digital Caviar Black 640 GB,Internal,7200 RPM,3.5" (WD6401AALS) hard drive, and am interested in either adding a 128gb SSD or adding another of the same HD. What my question is, what type of loading speed difference could I expect from a Raid 0 of that HDD vs. say, an OCZ Agility 4 128gb SSD? The current load times of my single HDD aren't bad as is, and in games I seem to load quicker than anyone else(I'm able to move before anyone else does after the loading screen), I was just wanting a reasonable increase in performance.
 
Raid-0 will not load your games appreciably faster. The doubling of speeds you see in benchmarks are by using synthetic programs driving overlapped I/O.

A modern ssd WILL load faster, particularly if you can attach it to a fast sata port.

I highly recommend a SSD. Intel and Samsung would be my preference.