I've built cheap PC with:
Proc: Pentium G3258 (overclocked to 4,2)
board: Asus H81M-E
ram: Kingston Value 2048 Mb DDR3
graphics: Asus Radeon 4350
PSU: FSP 400 W (that's more than enough for this PC)
case: cheap and small SilentiumPC
HDD: Seagate 500 Gb SATA 3
and that's very good configuration for MAME, Taito Type X games.