In Short, go buy a ballest that matches your watts and light.. Reason...Metal halide lamps have an operating voltage close to that of mercury lamps in many wattages, but have stricter tolerances for wattage and current waveform. Metal halides also usually need a higher starting voltage. Most metal halide lamps 100 watts or smaller require a high voltage starting pulse around or even over 1,000 volts.
175 to 400 watt metal halide lamp ballasts can power mercury lamps of the same wattage, but the reverse is not recommended.

Also, MH (Metal Halide) lamps come in three basic types:

*Base up (BU), which must be operated in the base up position.
*Horizontal (HOR), must be operated in the horizontal position.
*Universal (U), which may be operated in any position.

Orienting metal halide lamps in burning positions other than those specified can result in severe reductions in performance and potential nonpassive failure. Lamp life,light output and color can be affected by the burningposition. Some burning positions may need enclosed fixtures for safety reasons.

When replacing ANY HID (High intensity Discharge) lamp, never touch the bulb with bare hands. HID lighting gets exceptionally hot and any oils from your skin left on the bulb will burn and can cause the lamp to burst.

Always use soft, clean gloves when handling the lamps.

Observe extreme caution while foliar feeding any plants around exposed lamps. I suggest turning off any fans and ventilation while spraying the plants, any water blown onto an exposed, burning lamp can cause it to explode in your face. Be safe.

Although im no expert, I suggest you never miss around with electricity if your not sure what you are doing. Accedients do happen and nothing worst then geting your body found by your grow room. Or you burn your house down

Heres a cheaper HPS idea for ya.

How do I convert a common home security light into a remote ballast grow light? - http://www.drugs-forum.com/growfaq/1151.htm