Jpac will interface directly with a jamma connector. This includes monitor, joysticks, start buttons, coins, and 4 buttons each player. You have to wire more buttons to the supplied terminals on the Jpac.
The Jpac has a video amp built in, which is necessary for some arcade monitors who have too weak of a video signal. It does not account for the different frequencies of the monitor (arcade monitor wants 15khz, your PC is pretty much double that. The JPAC will BLOCK any frequencies that are damaging to your arcade monitor. You will need to either set your PC and software up to send a 15khz signal (soft15khz, advmame, dosmame, etc), or get an ArcadeVGA card, which will send the proper 15khz frequency. The arcadeVGA is the easiest solution, which is why you see people buying a JPAC and ArcadeVGA usually.