How much per month miners can earn from good PC? It will be profitable for them?
Roughly numbers - $5 per day on 1 GPU. But you will have to upgrade your farm with more RAM and powerful CPU.
For most miners, the motherboard supports 1 GPU per x16 PCIE (maximum 2 in SLI mode if there are two x16 slots on the board). All other PCIE connectors on the board will not provide the required data transfer rate for graphics. And the GPU in them will be useless. Therefore, the profit per day is $5- $10.
Can send the rest of the GPU to mining crypto-currencies, simultaneously with the game for 1 GPU, but then need the flagship i7.
If you put 1 GPU on the motherboard, depending on the motherboard layout and the size of the GPU there is a chance you have to remove another riser. But maybe you can move it to a M.2 slot if it not occupied.