As far as MAME is concern about video cards, it does not matter if you have lots of memory, but I suggest at least 64MB. Then you have to think about the other software you gonna run on this thing, also are you going to run any PC games with it ?
As far as the Video Card itself, I think get the best w/more memory just in case you want to run other that require more memory, 3d-graphics etc...As for the system for MAME, make sure you get the highest CPU and at least 512MB to cover most of the games in MAME.