It is probably not your video card that is causing the choppiness unless you have hardware stretch enabled. This will be the only bottle-neck on an older card.
MAME can be very CPU intensive, but it is not necessary to have a higher-end video card for any reason other that it doens't perform well with hardware stretch. In most cases, you can get around having to use HW stretch by manually setting the resolutions in the games.
If you wish to invest in a higher-end card and are using a VGA monitor, you should go with a GeForce 2 or ATI Radeon 7500 (again, you definitly DONT need anything more expensive/capable for MAME). If you're going to use TV-OUT, stick with card manufactured by ATI (don't get a knock-off based on the Radeon chipset)