Yeah, I'd be interested to know what an LX or XT disc brakeset is going to weigh (for front and back combined). You could always do it funky style and rock the mullet (disc front, rim rear). This is probably the most sensible (if not entirely sexy) way to do it because you get the groovy stopping power of a disc up front (where your power is anyways for stopping) and you don't have to pay quite as much cause you can go big and get a nice 40 dollar brake in back.