CVar gxRefresh

From WoWWiki

This cvar controls your VSync refresh rate - aka your max FPS while VSync is on.

This cvar defaults to 75, but is changed to match your monitor's refresh rate the first time you launch wow.

Possible Refresh rates:
 60Hz - Most LCD Monitors
 75Hz - Most CRT Monitors
120Hz - Special gaming monitors - in WoW, these are meant for use with the Nvidia 3D goggles.

You generally don't wish to change this setting as wow auto-detects this correctly.

Issues

As of 3.2.0, this cvar is bugged. The only possible setting you can use is 60Hz.