What size tank do you have?
Anyway, metal halides are HOT! Plan on top offs if using those, and have a decent amout of clearance. But, they will easily get you the wattage you want, and lots of usuable emissions. Also, power compacts can achieve it, depending on tank size. Power quads are pretty compact, about the footprint of a 10 gal, and 96 watts each.
http://www.hellolights.com/185cfquadret.html Search around that site for other lighting options as well. Some of the best prices AND quality you can find. There are other PC configurations to fit the lighting you want, depending on tank size, and they are quite a bit cooler than halides. I have some SW plants (macroalgae) that grow like mad under PCs.
And no you cant use on your current ballast. Unless you have a HUGE foot print you will not get that kind of wattage out of normal output flourescents,