Decide what monitor you want to use first. If you decide you want an arcade monitor, you can either go with the ArcadeVGA from
www.ultimarc.com or you can read further to figure out which cards available to you will run at 15khz.
If you're building the system for this, you're probably just going to find PCI Express cards. That is fine.
A 256 meg card will be fine if you're just running MAME stuff on it. MAME uses very basic 3D stuff for effects (like imitating the appearance of scan lines), which you're not going to want if you run an arcade monitor anyway.
If you want the build to be simple, you can probably use any video card with S-Video or Component out along with a TV and get a decent image. If you want an arcade realistic image, go with a true arcade monitor. If you want absolute plug and play run all games simplicity with no sweat but the least accurate arcade image, go with a PC monitor.
Each setup has its advantages. Hope that helps.