When I was your age I had a paper route, actually I still do and thats how I saved the $1000 to build mine. I would save more money, and learn to build the whole thing yourself. Take cisco or other computer building course if your high school has one. You can build a computer and not be the one who is S.O.L. if you jam a cpu in sideways. Arma 3 will be out this fall and will be even more demanding. Don't sell to gamestop because they are trying to profit, maybe craigslist instead.
I'll whip something up, do you want to reuse your case to put the money you would for a case into something else? if so what is the size of the case, like is it a thin slim, atx (7 pci plates on the back) M-atx (4 plates) or mini Itx (1-2 plates)