mbschs the WS467 switch is a dimable light switch made to work with incandescent lighting. I don't own any Metal Halide lights but I think they use a transformer or ballast type system for starting. The switch you want to try is a WS13A which is an appliance type module switch. This switch however is a Decorator Style switch.
If you do a search for "Metal Halide" you will find some other posts on these lights.
(http://www.bdshost.com/X10/lpup.gif)---(http://www.bdshost.com/X10/lpup.gif)---(http://www.bdshost.com/X10/lpup.gif)---(http://www.bdshost.com/X10/lpup.gif)---(http://www.bdshost.com/X10/lpup.gif)----KDR