Forget a whole PC, go after the key components, MB, CPU, RAM and GPU. Unless you're jumping major generation gaps, such as changing PSU connectors or going from PATA to SATA, everything else can be repurposed and reused. It's MAME, I doubt you're going to need a 10,000 RPM drive or the latest generation GPU or Gigabit network controller.
Edit: NVM, I didn't notice you wanted a higher capacity drive.