I'd try a better video card first. The 5770 is really underpowered for 5 copies of wow. It'll work, but that will be a big limiting factor. So much so, that the extra ram might not even make a difference. In fact, the GTX 260 that you have is faster than the 5770.

I upgraded to a GTX 580 from a HD 5870 and there is a BIG difference. No lag in SW anymore. I was getting like 15-20 FPS in SW, now I max out at 45 FPS (which is what I have my main window set to max).