upgrade processor macbook pro 13"

You can't upgrade the processor. It's soldered to the logic board and you would have to replace your logic board with another one which is compatible with the 13-inch MacBook Pro.

A GPU is a graphic card (in the case of the Mid 2012 MacBook Pro, the Intel HD Graphics 4000), and the CPU is the processor of your computer.

Respecting to the fan, it's normal that your Mac heats up when you are doing heavy tasks with it. If fans turn on automatically when it happens, there's nothing else to do. Your Mac is working correctly.

Respecting to Parallels, the problem is not how much RAM you can assign to the virtual machine but the GPU. The performance on a virtual machine is much worse when you want to do tasks that involve your graphic card. In this case, I would recommend you to install Windows with Boot Camp, onto a second volume of your hard drive. See > http://www.apple.com/support/bootcamp

No. You can't. Read https://discussions.apple.com/thread/5099738?answerId=22232789022#22232789022

Allan

Your MacBook Pro has got an integrated graphic card, so it's not possible to replace it

It would be a similar cost. Note that you have to find a logic board with a processor and a graphic card supported by OS X, and that it's designed for a 13-inch MacBook Pro. Impossible

If you are using Windows to play games, keep the drivers updated. Intel may provide updates that improve your GPU performance.

Also, don't set high settings in games if you don't want to make your MacBook Pro slow

You won't notice any performance difference unless you don't have enough free RAM now to run all the applications you need to use

You are welcome