There isn't much point to chasing newer gruntier video cards for GroovyMAME/MAME as it doesn't push their performance. Most people would just recommend something HD5000+ with analogue RGB outputs.
I've said this already in other threads (this is a very common question which I see pop up very frequently), but I'll repeat this here: newer cards buy you a better DAC which will give you a better image, crisper, more vivid colours.
It's not just about the grunt, it's about the technology used in translating the digital signals to analog.
I looked this up and Wikipedia says that RAMDAC (that is a set of 3 DACs built onto a chip with a little bit of RAM) speeds haven't really changed since 2006 (~400MHz with some exceptions). Also that DACs are essentially obsolete with "true color" modes and obviously with digital-only video signals.
The DAC needs to be fast, but all that really matters is that it is fast enough, faster than the monitor's ability to display an image. For 15khz CRTs using GM with super resolutions the maximum might be (2560*576*60) * 1.5 (allow for blanking lines) = estimated 132.71MHz speed required, well below 400MHz capability of most modern GPUs. After that any performance improvement regarding colour sharpness/clarity must be very marginal at best.
Maybe a VGA monitor could come close to the limit, but only at very high resolutions or refresh rates.
If you look around you'll find some discussion of this on overclocking forums, where they generally come to the conclusion that overclocking RAMDAC is pretty pointless in most cases, and the discussion just disappears in the era of digital displays.
So I just don't see that there is any way a faster DAC will make my blues bluer or my lines crisper, these things are determined by the limitations of my analogue display.
https://en.wikipedia.org/wiki/RAMDAC