I use a PC Monitor with MAME - not in an arcade cabinet.
I used to run 800x600@85hz, but that's just personal preference. (with a 17-inch monitor).
What size screen are you using?
Vector games will look better at higher resolution - recommend at least 1024x768 and more if your monitor will handle it.
I suspect the new MAME artwork (bezels) files will look better at higher resolution as well.
Refresh rate doesn't make much difference. Anything under 75 Hz drives me nuts for text, but I can't notice the difference in games.