I'd wait until someones gets a MAME port running on the Xbox. Your major cost is likely going to be a monitor. Given that PCs put out a VGA-type signal, you won't be able to use the existing arcade monitor. IIRC, most of those were NTSC, but they certainly weren't VGA. And since the Xbox has a hard drive to store ROMs and a TV-out, and it costs about $400, it's probably the cheapest solution possible. Of course, this assumes that someone gets MAME ported to it. Or the PS2, when it gets a hard drive. Or maybe a Dreamcast. You get the idea.