I'd say a Raspberry Pi B+, which is about $50 with the power supply and an 8GB MicroSD card, then stick RetroPi on it. That's what I have, and it works pretty well - you just have to make sure you get the right ROM versions for MAME, or it refuses to use them. It'll do N64, Nintendo, MAME, PS2, etc - pretty much anything you want. The new Raspberry Pi 2 might be even better, if you can wait a few days for them to become generally available. Space should be no issue with a Pi.
Edit: Definitely the Raspberry Pi 2. I just saw a video comparing speed between the B+ and the 2 for N64 games, and the difference in performance was dramatic.