Well, I'm anything but an expert, but I did just complete a mame project. Here's a link from this website covering soft15khz:
http://forum.arcadecontrols.com/index.php?topic=66402.0 It's amazing software and I'm fairly certain it's essential for having a computer display on a standard arcade monitor. I think your other option would be to use one of these:
http://www.ultimarc.com/avgainf.htmlI've never used one, but I've heard they work well.
My advice would be:
1. Do a bit of reading on both so you feel like you have a good understanding of them.
2. If you use soft15khz make sure you have a compatible video card.
3. If you use arcadevga make sure you have the appropriate slots on your motherboard.
4. Make sure the electrical wiring is appropriate as far as the isolation transformer for the monitor and taking care of the wiring to the switch power supply. Again, I'm anything but an expert, but I've heard that it can be bad to leave the switch power supply powered (I've also heard that it doesn't matter)...
Good luck (if I can do it anyone can). I'm sure others will chime in that are much better than I as this is a very helpful crew.
Chris