for that many plants in a space that big you'll be better off with one HPS lamp. you'll need a lot of CFLs for the grow you want. I don't think CFLs come in 500watt sizes....what you're thinking of probably is a 150watt CFL equivalent to 500watts of a normal lightbulb.
those 150watt CFLs are pretty expensive. by the time you get 2 or 3 of those you're already almost there to just getting a HPS lamp. Plus with just one lamp with a built in reflector hood it'll be much easier for you to manage the height and placement of the lamp. I just think for what you're looking for it'll be more cost effective not to mention you'll get a way better yield.

you can use a few small CFLs for seedlings though