Since I have a 400 watt HPS w/ ballast in the mail (this one specifically: 400 WATT HPS DIGITAL GROW LIGHT SYSTEM 400W SUN LAMP - eBay (item 270576653970 end time May-12-10 11:45:00 PDT) ), would I be able to plug a 400 watt MH into it like this? MH400/T15/HOR/4K 400 watt Metal Halide E39 T15 Bulb - eBay (item 350333377721 end time May-25-10 10:42:48 PDT)

They both use the same bulb cap type, E39.

Plus that would be cheaper and provide more lumens to the plants.

I just dont want to cause a fire.