Historically, WoW hasn't been GPU bound anyways. Not too sure about the latest expansion though.
I've been using my HTPC build as my main computer for the past while. It only has 4GB of memory too and all I can say is I have no idea how I survived with only that much memory in the past. Even my laptop, which generally only sees light use, is having difficulty keeping up, although part of that is the slower drive in it.
My HTCP is constantly thrashing the hard drive for swap. I currently have a healthy dose of tabs open in Chrome and Civilization 3 running in the background and it's murder. YouTube videos lock up, task manager takes forever to launch when I try to troubleshoot the issue, even the Win 8 start screen actually has to grind away loading tiles at times.
I have to strongly agree that bumping up memory is the top priority here.