Can’t find a good answer to this question even through a lot of research hopefully someone can give advice to this.
Specs:
Ryzen 7 9800x3d
Asus Tuf Rtx 5080
MSI MAG 321UPX 4k 240hz monitor
DDR5 32gb 6000MHz
No mobo yet still undecided
#Secondary GPU suggestion (and maybe Mobo) for 4k 240 hz monitor
undone flicker
chrome sky
GPU - check the max capability chart. RX 6800 can do 4K240 at 100% flow scale, RX 6700 and RX 7600 should do it at some reduced flow scale. RX 6600XT or RX 5700XT might still be able to do it at 50% flow scale (which is recommended for 4K). This is all assuming a base fps of 120 and running LSFG 3.0 in X2 mode without additional possible performance impact from HDR. If you intend to do X3 for 80 -> 240 or X4 for 60 -> 240 (or adaptive framegen where fps is around those numbers) you will probably need a next tier GPU like 6900XT, 7800XT, maybe even 7900(?).
You need a board with a PCIe 4.0x8 (or 5.0x4, technically) secondary slot. Any GPU from RX 5000, 6000 and 7000 series will be PCIe 4.0 as well. This is the cheapest AM5 board for x8/x8 in a good layout: https://www.asus.com/motherboards-components/motherboards/proart/proart-b650-creator/
The spacing between the full length PCIe slots is 3 slots, and Asus TUF 5080 requires four slots of space, so it must be installed on the second slot, and your case must be spacious enough with at minimum 1 extra expansion slot's worth of space under the ATX board installation area. Preferably 2 extra slots worth of space.
For more board options: https://docs.google.com/document/d/e/2PACX-1vQx7SM9-SU_YdCxXNgVGcNFLLHL5mrWzliRvq4Gi4wytsbh2HCsc9AaCEFrx8Lao5-ttHoDYKM8A7UE/pub
And of course, make sure your power supply is seriously capable if you intend to run 5080 with another GPU that needs 2x8-pin PCIe. I would have at least 1000W (and ATX3.1 spec for the 5080, naturally). A decent value recommendation: NZXT C1000