I have a Duron 850 overclocked to about 920.
I have a VisionTek (now out of business) 5632 GeForce2 GTS card.. Overclocked to the max.. Great card for $45 when it was still available before VisionTek went belly-up a couple weeks ago.
Key is: Be sure you have a motherboard that supports 4x AGP bus, and a video card that runs at 4X AGP.
Why? Because if you want any special video features like scanlines or special modes (or even normally), a 2x or 1x AGP just wont cut it.
Example: I had a Celeron 633 overclocked at 950 Mhz. Same card, different motherboard (only supported 2x AGP). I couldn't even run MsPacMan full screen without it dropping about 1 out of 30 frames or so.
Unthrottled, MsPac topped out at around 140 FPS on that system. On the new one, I get at least 350 if I remember right.
So motherboard + CPU + video card all go in the mix. You do NOT need a $250 GeForce4 to play mame games, because even with that card on an old sucky motherboard, you wont get any better results than with some no-name $30 video card.
So plan it right, and you'll have no problems.
BTW--Yes, even with the GeForce2 GTS (note: not the disabled "MX" versions), I can even play relatively newer games like Metal Slug full screen without a single frame drop. So $50 motherboard + $50 CPU + $50 video card is all you need... So long as you choose wisely!!!!!