Ideal light temperature spectrum for flowering is 2700 Kelvin and ideal for vegging is 6500 K. It seems that many metal halide bulbs sold for growing tend to be in the 4000 - 4200 K range which is probably fine for flowering but an HPS would be optimal. You can find HPS Conversion bulbs (360w I think) that will work with a MH ballast but they are pricey...$50 and up. Many people use MH (or HPS) for entire grows with great results so if cash is an issue, just go with what ya got and have fun.