If you're just using a PC monitor, pretty much any card should work fine.
If you're using an arcade monitor, either get the Trident Blade T64 (amazon sells it for $27), or wait a couple weeks until Andy Warne's video card comes out, it is supposed to be the holy grail of video cards for arcade monitors (
http://www.ultimarc.com). It costs more than the Trident, supposedly around $90, but Andy says it is in a completely different league than the T64. (Given Andy's involvement in this community and his great products, I would tend to believe his assertion.)
Oh, if you're using a TV with svideo, ATI cards are supposed to be very good. I think composite TV's are generally not recommended for MAME, so try to avoid those if you can.
Good luck!
-Mani