Trying to build a new computer

Well I'm going to school next semester and I determined that I'll have around 800 bucks left over to build a computer to use for gaming. But the thing is I don't know what's good in terms of the best for my money today and need some help picking out each part for it. Basically I need someone to build it for me and post links to all the parts I would need to have a really good computer for gaming in the price range of 800 dollars. I have a friend to actually build it for me when I buy the parts also.

Thanks again.