19" 4:3 aspect LCD monitor: $100
Mouse, keyboard: $20
Shipping (free): $50
That makes the PC itself $130. You could drop in a P4 3.4HT ($20 on the 'bay) in there (fastest HT CPU available for that chipset) and you'd have a mobo that would play/run 90% of the games available for MAME.
Forget about dual core, core2duo, and core2quad, they don't do jack squat more for MAME.
The P4 HT family is easily the best affordable intel processor series available for running MAME. (not including the i3/i5/i7 family)