Ever heard the phrase "You can't make a silk purse out of a sow's ear"? Your Schwinn World is a lower end bike -- nothing wrong with that, but no amount of part-swapping will make it more valuable than the value of the added parts. Usually it's much worse. Most bikes are like that - the parts are worth more than the whole. You could throw $100 worth of parts on there, and probably get another $15-$20 for it. The poster above is right - clean it up as best you can. Putting more money into it won't net you anything. (This assumes you are trying to flip it for sale. If you just want to upgrade parts so you enjoy riding it - go ahead, just don't think you'll get your money back out of it.)