I think what you're asking is what if you under-power your system, correct me if I'm wrong though. If this is the case, it'll either A) Not work, or B) Be quite unstable, resulting in crashes, etc.
Don't skimp on a PSU. This is one of the more important components, and is often overlooked. A crappy PSU can potentially destroy hardware, so make sure you have something that's of moderate quality at least.