You have 3 options that I can think of.  
First option:  You can use advancemame which works great but has a bit of a learning curve.  You will need a video card that is compatible with advance mame.  The older ati cards work great along with the trident cards, nvidia, and matrox g400 I believe.  Advance mame is very hard to get to work under windows so u will have to use linux or dos.
Second option:  You can use powerstrip.  There is a write up on how to use powerstrip, it's stickied under this forum.   Powerstrip also requires you to have a compatible video card if you are going to use custom resolutions.  I believe the ati and nvidia cards are pretty good to use with powerstrip.
Third option:  Happ used to sell a Universal Video Converter that would convert vga, svga, and xga signals to either cga or ega.  I heard it works well but the down side was that it costs $200 and is made by ultracade.
Personally I think advmame is the best option if you are willing to take the time to learn it.