> If I have a big enough cluster of computers, I can then generate enough proof of work in a cheap manner. Is that true?
Yes and no. Yes, you can get money doing this, but many people are. Because of this, the difficulty has increased to a point where it's fairly hard to make money (you've got to consider the electricity costs, for example). GPUs are the minimum you need, many people use FPGAs. Recently released are the first custom ASIC chips designed solely to mine bitcoins, which will drive up the difficulty extremely sharply and push GPUs and FPGAs out of the market.