Dell uses proprietary power supplies too. They also cheese out in other ways. If all you do is surf the web, email, and IM; name brand computers are for you. If you do anything that requires performance or you want to be able to customize/upgrade you need to build you own box. Its actually very easy and cheap these days. You really don’t have to worry about having to set dip-switches, IRQ’s, and configure hardware manually anymore. The best thing to do if you don’t know what hardware to buy is to read reviews at a place like Tom's Hardware. There's plenty of info on the web on building systems.
Click here for info on an emachines replacement power supply.
Cane is right on the $ with his first post. You should be able to get a case and power supply for around $50.