SSDs are primarily faster because they don't require any time to 'find' data. Much like RAM, any piece of data is accessible nearly instantly; versus a traditional HDD which must spin up and find any given piece of data on the disk.
The downside, is obviously the $/GB you pay for that extra speed. Typically it's most convenient to have a large HDD to store stuff locally; I wouldn't hesitate to steal the HDD from your other PC (assuming it's your old one) and put it in the new build to pair with the SSD.
Not everything benefits from the speed SSDs offer. They are very nice to boot windows from (your computer will turn on and be usable in about 20sec), and they will reduce the level loading times in games by a lot (for games that have long load times, like RPGs, this can be very very nice); however, most multiplayer games don't really see any benefit because you still have to wait for the server and the other players to load typically before a game begins.
I will be getting an OEM Windows 7, will I still be able to pair with the SSD? Not sure about how to do that.
I will be playing WoW, and as you said, I will be fine with load times in game, but I can't stand waiting 5 minutes like I do right now to reboot or physically load the game. It will be well worth the money if all of that is super quick now.
My wife will be taking my old PC, so I really won't mind transferring all my pics and video's I download into her PC (backup reason's as well)