If the supply fails (cheap ones die all the time) the rest of your components are likely to be fried.
What he said. Cheap power supplies ARE NOT WORTH IT. If you get a bad one it will most likely take out your whole system with it. Brands to avoid are things like Coolmax, Raidmax, Enermax, Xion, Apevia, and so on. I've seen what happens when you use cheap power supplies - it's not pretty.