Xavix :
My budget is 750$ (With OS, Monitor, Keyboard and Mouse)
I would definitely recommend looking up some gaming PC builds for your price range. Generally speaking, and especially for gaming purposes, more money should go towards a good GPU.
Now, depending on the type of games you intend on playing, the price of a GPU suitable for your needs will change a bit.
For very graphically demanding games, such as First Person Shooters, definitely don't cheap out here. I'd say you'd want to spend at least $200 or $250 on your graphics card.
If you intend on playing less demanding games, such as Minecraft or WoW, you could definately get away with spending about $150, with $120 being the absolute least.
The CPU is definitely still important, but for gaming, throw more money into a nice GPU. And if you're looking to get a good CPU for a bit cheaper, look at AMD's FX Series.
Good luck with your build!