I read you don't want to burn up all your energy with cardio before doing weights, or your fast twice muscles won't be able to do there max. But for some reason after a 3 mile run, I can bench press an extra 15 pounds. Maybe I'd get the same effect from a 5 minute warm up. The fact I don't have this figured out already is proof I have not been to the gym in a while, except for friday night, which is why my chest is sore now.
You should at least warm up before lifting weights. Warm up = about 5 minutes of fast walking. But like goingat20 said - do cardio on your non-lifting days.
Though, if your goal is to get big, too much cardio will suck up your muscles too. You ever seem a huge long distance runner?