These monitors are CGA, not VGA. That is why you just can't plug a VGA card into them without a hardware or software solution. You need something that will convert the frequencies/resolutions to an acceptable level.
If you buy the jpac, you probably also want to buy the ArcadeVga, so that it will automatically convert everything for you so that you can just plug everything into your computer and turn it on.
Keep in mind that if you want to add extra buttons (for example, the Street Fighter 2 configuration), that JAMMA will only allow a max of 4 buttons per player. From my understanding of the J-Pac, it will not give you those extra buttons that you would need. Of course, most other games will only require 4 or less buttons, so you're probably safe.
Note, I noticed die hard has three buttons per side, so if you wanted to add the fourth, in addition to drilling it in, you would also have to solder the connection to the correct part of the JAMMA connector.