Ventilation with a HPS is a must. Actually, ventilation overall (either HPS, MH or CFL (fluros)) is advisable... you don't want your plants starving for air.

There are two types of grow bulbs (step up from CFLs): HPS - High Pressure Sodium and MH - Metal Halide. I think your magnesium bulb is a Metal Halide...

There are a couple of people in this boards that do a lot of growing purely on CFLs... Wait for their input.

Hope this helps