What kind of processor do I need for example, a PDA?

I'm trying to know what kind of processor should I use to make a PDA, but a big one, a processor that uses a little voltage that doesn't need a cooler, or a fan on it, just like the actual PDA's that prescind of it, I've been searching trought the web, and I think that it can't be bigger that 1GHz and I don't know if they have a special name, so, please, anyone that could give me something, will be really appreciated. Thanks