Best bang for the buck is a 600W HPS. That's a bit too much power/heat unless you're serious about growing (proper air, filtration, circulation, etc). I have a 400W and it's perfect for about 4 plants. I use it on two, so it's very efficient.
On a side note, if you want to use CFL's, the 42W CFL's have the highest lumen/watt ratio, but still put off a little heat. A 23W CFL puts off little heat.