My card is a Quadro K4000 from a couple of years ago, so I’m probably better suited as a recipient of the wisdom posted here as opposed to giving any useful specific advice.
That said, here I go anyway:
The first thing to keep in mind is Cycles and the fact that is destined to become the builtin Rhino renderer sometime in the (near?) future. It loves GPU graphics processing units. The more the merrier. I also think that regardless of GPU, you should probably plan on no less than 8GB of GPU memory. John Brock recently posted some recommendations based on display resolution and number of monitors. I don’t think Cycles (or any other rendering software available today) makes any use of the ray-tracing ability available in the RTX boards. On the other hand, it is the Next Big Thing. I’m assuming Cycles will start using it as soon as Nathan gets it figured out.
As to Quadro vs Geforce: there seems to be lots of folks on the forum that are happier than a pig in a mudhole with the Geforces, All I can say is that I’ve always used Quadros and kept the drivers up to date. I’ve never had any display issues that weren’t the fault of Rhino (mostly WIPs). My budget has usually restricted me to low or mid-range Quadros and they have been more than adequate for what I do. But I’m not a big user of materials and textures.
Hopefully some users with recent experience doing more complicated stuff with bigger, more colorful and complex models using recent cards will have more details.