For CoD4 (obviously it's the main game you play, right?) the i3 would be the better choice as it fits comfortably in the budget and offers great performance. But I stick to Jacker's build. It may not be the best of the best per part, but all around it's a great system for your budget. The FX6300 is a great processor and from what I can tell it's the go-to budget processor. Coupled with something like an R7-260(X) you'll easily be playing CoD4 on max settings with great frame-rates.
Here's a build list I put together: http://pcpartpicker.com/p/3i37b a hair over $500 but definitely a good build. Even got a 500W PSU in so you'll have plenty of room to work with.
Also, you can get an OS for $20-ish. Not completely sure on the methods used for the people that sell the keys but if you're interested I'll give you the link in a PM. That's what I did to get Windows 8.1 for $15