I did some searching for you, but feel free to search on your own. This is from a WoW perspective, but im sure D3 is pretty close.

http://www.dual-boxing.com/threads/1...ighlight=drive

This has some pretty good info on it. But what from I understand is only wow goes on the SSD. OS writes and deletes to much that your SSD will kill itself. Make sure it dosen't defrag itself. Im not sure on how many times it can be completely wiped, but there is a number. I suppose this is why people opt for the lower GB SSD drives.

I don't see a reason for IS to be on your SSD. WoW loads a ton of files before you log in, and this is where you will more than likely see your gains.