Waiting for Andy's card is certainly not a bad idea and I will be getting one.
My question is what are you looking to do with your setup?
Using AdvanceMame/Menu and good old DOS, you can get that 15khz monitor to work fine with quite a few video cards. There is a list of cards that work with AdvanceMame in the build archive files. Some of them will even work with certain drivers that allow you to use Windows on that 15khz monitor.
When I first became interested in getting my 15khz monitor to work I tried all of the cards I have in my collection. PCIs, AGPs, Cirrus, TNT2, Banshee, etc. Most worked with varying degrees of success. After reading up in the forums I decided to try a Trident Blade T64 which I found for about $30 USD. It works great with all the apps I've tried, has a lot of available modes, is plenty fast and has 15 khz driver support for DOS and Windows.
Where am I going with this? Before you buy anything else (new cards, scan converters, etc.) give the stuff you have a shot. Read as much as you can take in, and the knowledge you gain will be valuable regardless of what ultimate goal you have for your Arcade project.
And don't feel bad about having a 15khz monitor. I have run some of the VGA games (Prop Cycle comes to mind) and they look OK. The biggest problem I have with the VGA games is not the look on the monitor - but they seem to take a bit more horsepower than my current computer system can deliver.
George