The problem is that your Xeon is on a server class socket, and it’s Sandy Bridge. So it’s not easy to get a meaningful upgrade, and it’s a pretty slow platform anyway.
Something like an E5-2643 should work on your board and is only a tenner on eBay. But it’s false economy really, when Haswell is also cheap and such a massive improvement. Unfortunately you’ll then run into the AMD thing anyway. Their Windows drivers for OpenGL (which Zwift runs on) are pants compared to those from Nvidia.