How on earth you have managed to get that sort of speed on your current RAM is beyond me.
The jump in speed should only be noticeable if you're using the ram, it allows for a lot more multitasking, and for overclocking as well (but you'd really want 1600MHz ram to do that with). Generally I don't think you'd see a straight up improvement just using WoW, but if you had WoW open with youtube, skype, and irc and everything open, you'd probably see it improve.
I'm not gonna say Alienware is shit, but Dell are a nightmare to work with
I'm sure you could build it for a lot cheaper and stuff a lot more stuff in there, but that's 100% up to you.
Good luck with the new computer