I've currently got my Mame/emu machine set up with a P4-2.8, with 1g memory and a ATI 1650 pro card, and it runs almost everything in mame fine, as well as most of the older system emus. I just upgraded my main PC from a AMD 4.6x2 to a 6.4x2, and am toying with the idea of moving the 4.6 into my Mame machine for other system emus (mainly the newer console emus, and possibly the newer games in mame that my existing system doesn't have enough HP to run). I've already got a board/ram, etc for the 4.6 (a Abit an-m2hd, has Nvidia 7050pv graphics, hmdi out, etc on board), and 2g of ram for it. It's got a PCIE slot so I could upgrade the video to some thing a bit more tube-tv friendly (if it had component out, I'd be set, my TV has that, just no hdmi or DVI). At this point, the only part I'd need to buy at all would be either the HDMI convertor (expensive!!), or a video card of some type, PCIE, with a decent GPU on it.
What I'm wanting to know, is the proc I've got really overkill for my EMU (it'll really be running primarily older MAME stuff, but I'd like to have options down the road), or maybe use it somewhere else around the house (maybe upgrade one of my HDTV PC's, but the ones I have seem to run perfectly fine at this point)...