Quote Originally Posted by Opie Yutts
It really is to your advantage to get a MH bulb as well, since it is better for vegging. Your plants won't end up as lanky and will reward you with more nodes.
I can't second this statement enough. Before I ever bought a conversion bulb for my 600w system, I grew both phases with an HPS. I got some horrible stretching early on, which always gave me tall, lanky branches...and nowhere near enough nodes. I also generally stick with indica-dominants, which are supposed to stay squat and compact...yet I generally ended up with 2' tall plants, with minimal branching, before flowering. Once I popped in a MH conversion bulb, this changed instantly...and my plants finally took on the classic characteristics of vegging indicas. I was finally able to get some density in my buds, too.

KB: Get you a conversion bulb. You won't regret spending the extra money on one. That, you can take to the bank.