ArchiCad itself doesn't really use your GPU for rendering, instead, it uses your CPU, so if you're rendering straight from ArchiCad, your GPU does very little work. For this you need lots of fast cores in your CPU.
Real-time rendering, however is a different matter altogether! Twinmotion is what we use, so that's what I'll talk about 🙂
Twinmotion is built on a gaming engine (the same one as Fortnite and Rocket League, among many others), but unlike when used for games, the engine needs to render thousands more polygons for renders than when gaming. This polygon count uses MUCH more vRAM from your GPU.
How much vRAM is enough depends on how big your projects are and therefore how many polygons you need to render.
So is a 4090 amazing? Yes, mostly because it has lots of vRAM... is it needed? For large projects yes... for residential projects... no.
We do multiunit developments running on our old RTX2070 Super (8Gb) and it runs just fine. So you've gotta be doing something pretty big and complex to warrant the 4090 beast!
If you are already powering in with an i9 13900k (or ks), 64-128gb ram, good z690-z790 motherboard chipset, M.2 storage, and a good power supply, and there's still budget left over for a 4090 go for it, but if not, 12gb or 16gb gpu is plenty unless your rendering the largest and most complex scenes!!
And you'd be better to get two 4080's for the same price as one 4090, as Twinmotion is now capable of splitting the load between two GPU's (and with the 4080 you'd get 32gb of vRAM rather than 32gb)
Hope this helps
MickeyNZ
AC 13 - 27, Twinmotion
Windows 11 pro
i7 10700k, 2070 super, 64gb DDR4
