Hi Riffing,
Welcome to the forum. Yes, PCI 3.0 is backwards compatible, so it should give you no problems.
But regarding your new build..... it's time for a nice graphic card upgrade
😛Greetings,
Remco
AMD 5900X
Asus Crosshair VIII Dark Hero
64GB G. Skill 3600Mhz C16
Cooler Master Master Liquid PL360 Flux
Be Quiet Dark Power 12 Pro 1500Watt
2 x Samsung 980Pro 1TB
1 x 10TB WD Black
1 x Asus Strix RTX 3080Ti OC
CoolerMaster C700M
Samsung G7 Odyssey 32inch