You definately want to startoff at 640x480 if you can. You can set windows to do this with in a few ways.
I believe your monitor will hande that right?
(xp right?)
1) right click on screen, properties, Settings, Advanced, Adaptor, List All Modes, find 640 by 480, True Color (32 bit), 60 Hertz
Or you can install quickres. This is a bit easier but will have all the same resolutions (maybe more?)
But both will give you more options then you really want. There is an application out there somewhere that will remove resolutions you really don't want. But there are a TON of these and not really worth it.
But once you pick a resolution here, it will come up in that resolution. NOTE: you will ALWAYS hit 640x480 on the initial boot window.
btw, I'm running 640x480 on my 27" benson on all vector games and punchout (dual monitor games). Then I'm forcing 640x240 and hardware stretching on everything else. Almost all games look great. But doubling the resolution for vector games is a huge deal, but isn't 15hz but vector games aren't 15hz.
I think I could get rid of the hardware stretching, but I would have to remove a bunch of other resolutions. It kept trying to go to the 50hz games that made mame screw up.
I would love a wiki for mame setup with specific hardware. I know a bunch of people have the identical hardware as I do (and very close to what you have) and it would be nice to see whats working for others.