I've had a few successful grows now and I haven't used anything else but my 250w hps in a small cabinet. I think the sodium light is awesome for early veg. I've noticed that my mature plants look a bit smaller than they could if I vegged under whiter light. anyway, don't even worry; they will grow sublimely under an hps alone even from the seedling phase. if budget is an issue just go with what you have and any upgrades in the future will yield somewhat better buds.
just make sure your plants get fresh outside air and keep temps under 85 F