The AVGA5000 is based on a Radeon 5450, I believe, and you can get waaaaaay more power out of something newer, which won't help in MAME because MAME is all CPU. Anything from the R9 series should be great for Demul or Supermodel though. A DVI-I port with a DVI-to-VGA adapter is the same as a VGA port. The adapter is super cheap, obviously, and the DVI-I port is easier to find on newer or more powerful cards. Not every GPU listed as being supported necessarily has a DVI-I port though, so make sure to check. They often look the same as a DVI-D port though they're not meant to, which is digital-only, and won't work.
Any GPU that has more than one fan on it might mean you need to actively vent the cab, if the cab itself doesn't already have a fan. Passive venting probably won't be enough.
@Mitsurugi - I'm having some V-sync issues as well. Let me know about any fix. It's hard to search for V-sync on this forum as it's also the term used for Vertical Sync as part of the analog video signal. I know there's info somewhere about interlaced modes running at half speed, but I can't find it, and anyway I thought it had been solved.