This is turning into an interesting thread. I am out of my element on arcade monitors, but . . .
First, I don't think you need a J-PAC unless you want to run the original arcade boards, which you wouldn't to use MAME.
I think you can get by without the ArcadeVGA card, but it would make things much simpler.
Just b/c your panel has only two buttons, you could add more buttons for the cost of a 1x6x3' ($5) and $1 each for buttons and wire them up to the KeyWiz, so you're not really limited to NES games.
Also, it should be obvious, but see
http://www.oscarcontrols.com/monitors.shtml There is a big difference between a PC monitor image and an arcade monitor image. If the PC is the look you want, maybe you need to go that route first, as you'll never get the same crisp images out of an arcade screen. If you want the realism of an arcade screen, you will want the NES screen you already have and won't be happy with any PC monitor, so don't bother with that.
I guess I need some more info . . .
Does the P133 have a basic monitor hooked up to it now? What about a hard drive? Do you want to keep any of the P133 as a basic system or can it all be scavenged for the arcade cab?
Assuming yes, yes, and can be scavenged, I would recommend the following:
Get either an old version of MAME (pre R36B12) or Vantage (?) and load it on the P133 and play some games and see how it performs on the games you want to play. This should help determine which way to upgrade.
Assuming it is too slow, look into and invest in the following -
Find out what kind of video card works best with arcade monitors, seems like I heard Trident Blade 3D, or ATI RAGE 128, but search the monitor/video section of this forum and you should get an idea. These cards should be under $20 dollars from E-bay or
www.newegg.com,
www.pricewatch.com, etc. DO NOT get on-board video, as it will be hard to interface with an arcade monitor.
Get a Duron 800-1.0 GHz processor for around $40 dollars, or a XP1500 (better) for around $50.00
Get a CPU fan for it.
Get a basic mobo (KT133A or so) vintage, for around $40 from either E-bay or
www.pricewatch.com.
You should be able to get a board that supports either DDR or SDRAM in this price range, but SDRAM is as expensive as DDR now and DDR is faster so . . .
You should be able to find 256M of PC2100 (PC1600 would work) DDR for around $25. 128M would be sufficient and be a little less.
I realize I took you to around $130, but you should be able to play about 50 % of all MAME games with this set-up (more than that if you add a trackball, or trigger sticks, or spinner, or . . . you get the idea, down the line . . .)
Good luck and post back if you have more questions.