AJ I can type more later or maybe get some back and forth in over Facebook. fake edit: this ended up being a wall of text so maybe this is enough to get some gears turning.
So this is all 15khz gear it seems. I see a few schools of thought here.
Almost most flexible and highest quality way: OSSC (hardcore gaming oriented linedoubler/linetripler, outputs DVI + analog audio) + HDMI capture card (HDMI and DVI use the same video signals, HDMI just adds on digital audio as another signal + a bit more). I've seen the results of this both in person with KevinDDR aiding in arcade capture at Arcade Superplay Expo, and in KevinDDR's own stream, and I swear to God it looks like an emulator capture. OSSC is fantastic at its digitzation and somehow makes things look pixel perfect, sampling is just incredible at making things sharp. Hats off to that wonderful engineering. About the card, HDMI is a digital signal and there's not much room for a capture card to go wrong here, you're no longer dependent on the capture card's ability to digitize analog (more functional than beautiful usually and surely not game-optimized in nearly all cases), same concept as using an external scaler on a TV really, only the OSSC is made to really kick ass at 15khz and up RGB/YPbPr games. So just get anything that people say works well with 480p and or 720p HDMI and you should be fine. OSSC doesn't convert framerates so if a card is super strict about incoming refresh rates, this could be a problem, but from what I've seen things not working is the exception. NeoGeo and CPS2 aren't far off anyway, compared to some of the weirder stuff (like Cave). As a bonus you can use the OSSC to play RGB/YPbPr consoles on LCD with NO lag, not exaggerating here: it's behind realtime by 2.5 scanlines. Since the next frame doesn't exist at that point it's lagless.
More flexible and barely lower quality way: XRGB Frame Meister (hardcore gaming oriented video scaler, outputs HDMI) instead of OSSC. The Frame Meister can output TV-friendly framerates so that's one advantage it has on the OSSC, you shouldn't run into a board with weird output that this thing won't clean up. You might even have a Frame Meister already, they were pretty popular for a while, and as above you can find use for it outside of just capturing (though it has a few frames lag).
Ghetto method to complement the above: Gonbes VGA scaler + VGA capture card. I know the Gonbes is built to a price but I'm just not a fan, I'll be honest. TBH I don't see the positives to introducing that weird Gonbes into all this and building a setup based off of that, unless you happen to have a VGA capture card already, in which case that's all you'd need. That said it might be good to keep in your bag of tricks if you have a very strange board that you can't capture in analog (below) but that this could clean up, it'd be heaps better than nothing. And if you actually get mileage out of it for playing off of (maybe the slight Gonbes lag and VGA in on your TV lags less than a direct connection from a console) then so much the better, but they're so cheap you could just get one to play with without breaking the bank anyway.
Now let's forget scalers and stuff and try capturing the raw output, or at least 15khz analog...
Another way: card taking analog RGB natively which takes arcade signals (ie doesn't start at 31khz). There's cards using I believe a Yuan chipset which can do 15khz, 24khz, 31khz, up to 1080p. I think this is all of them, it's at least all I know of:
Startech USB3HDCAP
Startech PEXHDCAP*
Startech PEXHDCAP60L
Micomsoft XCAPTURE-1
Micomsoft SC-500N1*
Micomsoft SC-512N1-L/DVI
*I think these top out at 1080p30, the others are OK at 1080p60.
I use a USB3HDCAP and it has a few quirks but they're easy to solve. Drivers could be better and there are pros and cons to certain versions, but usually you can use a slightly older driver that does basically everything and that'll be fine. I've captured 15khz, 24khz and 31khz RGB with this and like the results a lot. There are a few cards out there such as BlackMagic's stuff that can take in 15khz RGB, but only as interlaced video, so that sort of stuff is useless for arcade stuff.
And finally, a bit ghetto but not bad variation to the above, the oldschool method: encode RGB to NTSC or YPbPR with a JROK/Neobitz/CVS 287/whatever and capture that. I used to capture my NeoGeo with a supergun's S-Video out (some weird Sony board that Matt Ross hacked up, that dude was creative) fed into the S-Video jack of a Hauppague ImpactVCB, powered by the bt878 chipset. That supergun has changed hands a few times and ended up being used for capture at some of the Games Done Quick TGM showcases, so if an audience of millions can view it and think it's fine then it should meet just about anyone's bare minimum standard. If you spent some time tweaking the proc amp just a bit you could really make use of all S-Video was capable of. I've also used a RGB splitter and a YPbPr encoder (CVS 287) to capture in Component with a Hauppague WinTV-HVR-1500, results were OK but I think the CVS is just a bit crappy (I barely paid anything for the CVS and the card so I didn't really care). Anyway, if you have an old card lying around with analog TV inputs then you could do this cheap and still get pretty good results. You'll need a card that works with noninterlaced video, I can tell you from experience (I worked there lol) that ViewCast/Osprey by Variosystems cards are strictly dependent on the incoming video being interlaced and will just show No Signal when fed noninterlaced (240p) video. Also, I believe Hauppague PVR series USB cards can do 240p over Composite and S-Video but it won't work over YPbPr, going from memory here.
Addendum: Splitting JAMMA RGB
You wanna keep it simple? Either make a male-female JAMMA passthru board that splits RGB off via some resistors (I used 300ohm in my beatmania), your monitor's video will dim slightly, or if it's more than slightly then tweak brightness a little higher. Or more ghetto than that? Skip the JAMMA adapter and hardwire a split into your JAMMA harness, you can even use some vampire clips or something similar if you want.
You wanna not keep it simple? T off video to a viletim AV driver:
http://etim.net.au/av-driver/
Your cab's monitor won't be affected by this thing's output being hooked up to your card, but this is a more complicated install. I'm not gonna elaborate, read the website, and if you can't figure it out this probably isn't for you.
What will not work here is a VGA splitter, from what I've gathered talking to system11 they are dependent on the 0.7v p-p voltage for normal video signals and JAMMA video (which is operating at a higher level) doesn't really work with it at all.
