Are you going to salvage parts from the old PC, or do you want to keep it running?
2500K for sure; you picked the two most CPU-intensive games around.
You don't have the budget for SLI/Xfire at the moment, but we can build you something that'll let you add another card in the future.
$125 ASRock Z68 Extreme3
$100 Hitachi 500gb HDD
$35 212 EVO
$20 ODD whatever
$60 case? Trawl Newegg and see what catches your eye; most cases work fine. Just check here.
Try the Apollo, Challenger, 912 and Antec 300.
That's $560 without PSU and GPU. This is looking a little tight, especially if you want a PSU that can handle SLI.
Corsair TX750W, $90 after rebate.
$160 for a GPU? That's kinda lame for a 2500K build, and it certainly won't come close to maxing BF3.
You can get a 560 Ti for $190, and two of those will almost max BF3. One will do you quite well on high settings, and you can add another later to catch up to the latest games.