You need a computer....
motheboard, processor, ram, hard drive, *network card optional*, sound card *not necessary if built in*, video card *ATI Radeon 9700 made by ATI w/ the s-video out, VERY good card for mame*, a 300w power supply unless you're going to have a lot of USB equipment, if so get a 350w - 400w power supply.
try looking at
www.maincomp.com 's build your own computer section.
this is way overboard, but this is what my friend's computer sort of looked like *we got rid of some of the stuff and bought cheaper stuff elsewhere, like I got him a cd-burner/dvd-reader, and a cheaper video card cause they were out*
($89.99) AMD Athlon XP 2400+ Socket A 266FSB 2.0GHz
($79.99) ASUS A7N8X-X Socket A, AMD ATHLON/DURON/...
($67.99) 512MB DDR PC2700 333mhz Memory
($97.99) SPECIAL EDITION 120GB 7200RPM Model #WD....
($37.99) Samsung 16x DVD-ROM Drive Internal EIDE OEM
($65.99) Chaintech Geforce FX5200 128M DDR
most definitely overkill, and this is not including the case or the floppy drive.