Hi there!
First of all, Minecraft does not challenge the GPU at all unless you are using any HD texture packs, or other graphical enhancing mods. Minecraft primarily uses the CPU for terrain generation and rendering.
As far as why you were getting a higher FPS with your 650, I'm not entirely sure, but sometimes the oldder GPU's can be better at direct computing, whihc would mean that the GPU would basically be acting i part as a CPU. Maybe this is the reason, I dont know.
But to be honest, no matter what GPU you have, your 8120 should be more than adequate for well over 200FPS, so maybe it is another issue? Have you underclocked your CPU or locked any cores by any chance? Long shot I know. How much RAM does your system have? What speed is it running at?
Another thoery could just be that there are some Nvidia Files still left over, even though you uninstalled them. I would suggest manually checking just to make sure, and also running a registry check and defrag. If need be, manually edit the registry if there are files left over.
Have you perhaps got a virus on your PC that could be causing it to run slow?