Having an SSD will significantly reduce your load times but will have little effect on your overall performance. I can load a single instance of wow into dalaran in about 6 seconds compared to about 20s on the same system loading from the hd. Loading times into instances is basically instant. But like I say ssd will do very little if anything to improve your fps.

Do your homework not all SSD's are the same. Some of early cheap models (ocz, samsung) actually have lower real world performance than regular HD's. Intel M series and the OCZ vertex are two models that I know of that are good, there is a reason why they are $100 more than the rest. Don't bother with raid unless you got money to burn or won a lottery recently.