I'm using a 22" 1680x1050 (master) and a 24" 1920x1200 (slaves) screen for my boxing. I think the important thing is same aspect ratio (for fiveboxing at least) so you don't get stretched or compressed views when switching.

alas i read somewhere that it is better performancewise to have the same resolution on both screens. i won't put my hand into fire for this, though.
6ms is not refresh but switching from gray to gray (presumably) of the pixels (in german it is "Reaktionszeit" which translates word by word to reaction time.
refreseh rates of LCD's should always be either 60 or 75 Hz. (or 120 or 200 for extreme fast 3d displays or something, but mostly 60 or 75). but even if you get a faster one it should be able to display at 60 Hz so they both can use the same vertical sync which is again important for performance i think. don't quote me on that one either ^^
imo buy a decent brand and go for what you can afford without hurting yourself instead of looking for a 2-4ms one. those extreme stuff really seems to matter only in shooters. if you play shooters, too and do so on a very high level you might want one of those fast displays, for wow i don't think it matters that much...