I haven't done any CPU mining with Monero yet, only GPU mining. However, based on what you told me, you may want to check if your parts are compatible first. This is a handy website to guide you: https://pcpartpicker.com/

To see if the Vega GPU fits, that would mainly depend on your motherboard. In terms of mining, I wouldn't recommend mining XMR with only 1 Vega 56 GPU. I don't know the exact numbers for the Vega 56 but it seems that it'll be around 800-1000 H/s, which is about ~$1.50/day. IMO, you're better off investing (unless of course you're buying the GPU for other reasons besides mining).