I say always use a weight that alows you to complete a full range of motion. (Unless your are purposly trying to do partials). I don't ever see a need to go that heavy on an exercise like peck deck though.
work up to the weight by using good form w/ a weight you can handle for reps. how many reps could you possibly be bangin out if you can't fully do 1 rep. You shouldn't be goin to failure with every excercise and every set, especially an excercise like this one.
the reason im using the peck deck is because i broke the bench press on the multi gym! (i snapped the handle off when i attempted the whole load one handed...one of my genius ideas!)
so i have been doing dips,leaning forward to work my clavicular head, and backwards to work the sternal head. but im unsure about the effectiveness of these as im only lifting my body weight. will my muscles get bigger or am i going to have to invest in some free weights for doing bench press?
Yes, you should consider the free weights as you will need to continue to overload the muscle in order for it to grow.
Bodyweight will work but only to the point where you are able to do up to 12 consecutive reps, once you pass that, you are probably not over loading the muscle any more.