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.
I think the main bottleneck for people who want to use their PC for both Playkey mining and other normal tasks (eg. surfing the internet, doing work, or even playing games yourself) is the CPU. A lot of games can require the utilisation of a lot of the CPU, and you can't add an additional CPU to the system. (You can add more RAM, GPUs, hard drives relatively easily).
The profitability needs to be decent, or else almost all of the 'miners' might just be PC farms (which is centralizing things in a way... but it might work out to be the best for everyone).