New Pc build Help - Limited Budget

I want to get back into gaming and would like a new pc to be able to play games at 1080p on normal settings. i dont need a system that can max all games out with 100fps. i have about $300 to spend on the components in the tower. my problem is i have an imac with a i5 2400s in it but the gpu is lacking, thats why i need a gaming build. Could i take out the cpu in the imac and use that towards the new build? if i cant i an willing to use a slower cpu and add a decent gpu to play the games. i really like the monitor it has too but i dont think i could use that. if im wrong please let me know.

i already have these components on hand os, monitor, kb+mouse, hard drive, disk reader, sound system.

i was out of the scene for years. thank you for taking your time to help someone out.

Can't do that with 300$

No, you cannot remove the i5 CPU on your iMac and install in on PC.

http://pcpartpicker.com/p/8rc7qs enjoy

bought and thanks