Well...as high as it can go really... depends on your monitor. Try the highest and then go down in increments until it displays at the selected frequency.
Playing WOW on Vista Enterprise x64. No problems at all. And playing on VISTA doesnt make any difference, except mayb a lil slower bt i havnt seen it with my setup and graphics are the same VISTA or not. WOW gfx engine definitely needs an upgrade
VSync just prevents horizontal tearing. If you start to see tearing when you look around quickly, turn it back on. It does cap your framerate to your refresh rate, but that's at least 60, and you don't need more FPS than that for a completely smooth game.