I'm running a dual core 2.53GHz, 3GB memory, 9800GT and have absolutely no FR or lag issues. My main window at 1280x960 runs 40-50FPS, and still gets 25 or so in shat, usually 30-35 in org. I have my clones (4 of them) set to 800x600 each, 10fps in the background.

I run the main window on CPU0, 4 clones on CPU1. The only downside to this is that if I have to switch to a clone window its a bit laggy (clones don't lag following my main or anything, just making one in the foreground or PiP is not very good), but if I move 1 clone back to CPU0 with the main, the clones are fine, and the main is fine, but it drops the FPS of the main by 10 or so, so 99% of the time i'd rather have the clones on a separate CPU.

Anyway just pointing out that I have a way lesser system than most people in this thread and dont have any lag or FPS issues. Maybe you guys have the detail settings way too high? WoW doesn't really look that much better with everything at max compared to everything at min (other than view distance which is nice to turn up a bit), so theres not really a compelling reason to set your graphic details high if they drop your frame rate too much.