Give a look at here:
www.tomshardware.com/t/build-your-own
en.wikibooks.org/wiki/Buyer's_Guide_For_Building_a_Computer
www.tomshardware.com/reviews/build-your-own-budget-gaming-pc,3780.html
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Focus on AMD video cards, they offer a better performance/cost ratio, which makes them preferable to NVIDIA ones for limited cost builds.
Sapphire is the best AMD video cards make, Asus is very good too.
I would choose Windows 7 (64-bit version of course) Home Premium. Windows 8/8.1 is weighty, and should be avoided on non-expensive computers such as the one you are planning on.
Besides, it brings compatibility issues with not a few non-up-to-date games.
Intel Wi-fi cards are probably preferable to Asus', Asus one being good too however. Choose Intel if a difference like $10 does not bother you.