What he said.
Remember the ArcadeVGA card it used to be able to interface the PC to the arcade monitor, it has nothing to do with the controls.
The J-PAC is the encoder for your controls. It plugs into the JAMMA harness so you don't have to go re-wiring anything and that way you can still use your JAMMA PCB's. Although, as the JAMMA standard is for 2 players and 3 buttons, if you want more than 3 buttons per player you will have to wire them to the extra screw terminals on the J-PAC (it accomodates for up to 8 buttons each for player 1 and 2). The J-PAC does have an inbuilt video amp as well to boost the signal from your video card to the monitor.
For a 4 player cab though you are going to need more than a J-PAC. You can either forget the J-PAC and go for an I-PAC4 but then have to rewire your entire control panel and wire to the monitor seperately, or stick with the J-PAC for player 1&2 and use either an I-PAC2 or Mini-PAC for player 3&4. With the J-PAC and I-PAC (or Mini-PAC) you can still use your machine for regular 2 player JAMMA PCB's (and 4 player if you wire up the connectors right).
With my 4 player cab I have an ArcadeVGA card, J-PAC and Mini-PAC. With the ArcadeVGA and J-PAC you can have MAME up and running within a few minutes in a working JAMMA cab. Player 3&4 takes a little bit of wiring and programming of the I-PAC/Mini-PAC. I went for the Mini-PAC because I really didn't have to do much wiring at all as I got the one with the wiring harness included. Additionally the Mini-PAC also supports one trackball and one spinner in USB mode so you wouldn't need to get a seperate Opti-PAC.