Amount of wattage?

[Wilty]

Oh my god, my brain hurts. Bouncing around looking at website after website. Heat aside what am I do be looking for in a ballast? whats most important? I think I should avoid the ballasts from work now and buy the ballast that matches the bulb at the store. But thats sooooo much more money. I can get a 250W ballast for 70 bucks.