I have an SSD drive. It's OK, but honestly, I didn't notice much difference. I think a well tuned system with a solid HD (velociraptors) will perform very close. Having enough RAM is the key to not making the HD a factor.

What I'm planning to do is get another 12G of RAM and just load wow into a RAM drive. I actually don't think this will matter that much, but 12G is pretty cheap, so what the hell.