Need help buying CPU

I want to buy a new computer (desktop) but I am stuck!
I can decide what CPU to choose. Intel or AMD?
Core 2 Duo or i3-i5?

The computer will be for general use but I also want to be able to play a couple of games like Left 4 Dead or Crisis.

Right now I have 500 Euros and I already have a monitor, mouse and keyboard.

What do you suggest