Well, if you exercise until you can't lift 50 pounds for another rep, you've fatigued your muscles more (deteriorated more myofibrils) than if you exercise until you can't lift 180 pounds for another rep, although you're not going to build your fast twitch muscles as much only doing low weight.
... and you are quite possibly just overtraining, leaving yourself worse off than before!
Although I've never used it myself, I know guys who swear by the method of working with high weights to depletion, then lower weights, then lower, until they fail to lift a fraction of their maximum. Whether your muscles can recover from that in a timely manner depends largely on the kind of condition you're already in, I wouldn't suggest trying it if you're not already a veteran.
I'm looking for resources on effective weight training for the purpose of physique building. It's an area with a particularly poor signal to noise ratio so I would value pointers from other rationalists. The kinds of questions I would like to answer are:
Edit: I'm vegetarian, and I now realise this is rather important to answers to point three. So far the only supplement I've been taking is soy protein.