Building PC...need advice

My little brothers computer went all crazy and I don't feel replacing parts since it's already pretty aged. Even before his computer went on the fritz he wasn't able to run the games he liked playing too well. So now I want to build him a new PC for christmas but I'm having a hard time deciding what parts to put into it. My budgets gonna be around $1300 and I need it to run current games decently. It doesnt have to be a crazy "alienware" type pc. As long as he can play his games with good fram rates. Examples of his games are Left 4 Dead, Killing Floor, Call of Duty series, Team Fortress 2, and almost anything Half Life. Thanks for any help given.

You're still pretty far out on your timeline. Two of the major components for a gaming build are the CPU and the GPU. Since we're due for the release of the Intel i5/i7 processors and the LGA1156 motherboards (already for sale in Taiwan) along with the new ATI 5850/5870 DX11 graphic cards before you'd do your build, i'd suggest you wait and keep an eye on the new developments. As for cases i really like several of the new Coolermaster cases. A good place for reviews is here
http://www.hardwaresecrets.com/page/case
Keep doing research and when it's time to buy and assemble, you'll be well informed and get the biggest bang for your buck.

I'd do research now and keep up on the new tech that comes out as dm said. But if you don't have plans at 3am EST BLack Friday, I'd start you're shopping then. Some crazy deals even here on the egg, but they go quick. Last year, the mailer was sent out 3 hours earlier.