Oh, I love being able to show off all my dream builds in these kind of threads!!!
You need a CPU?
Core i5 2500K is the no brainer for an intel fan, easily overclocked and unlocked (hence the K) you can get this baby to 4GHz with a good 3rd party cooler. Besides it outperforms all but the Phenom II X6 1100T and i7 2600K. Best value for the money and top choice for most benchmarking tests these online magazines run on the latest games.
You need a Motherboard?
I reccomend the Gigabyte GA-Z68-UD3H, great features, and full support for the Sandy Bridge Processors, it has good overclocking utillities, UEFI instead of BIOS. Plus if you buy at Microcenter (BEST STORE) you SAVE $150. CPU and Mobo together will cost $269.99.
You need RAM?
Well, most of this is gonna be similar so... but I reccomend a stick of 4GB DDR3-1600 ram, even for BF3 you shouldn't need more than that. I reccomend the Vengeance Series from Corsair for looks alone. Around $30-50.
Now that leaves us with on the high end $290 left over in a $600 budget, but more likely it will be around $300-310. Throw in $50 for a goodd 3rd party cpu cooler (i.e. ThermalTake or CoolerMaster) and you have around $250.
So you can afford a beast graphics card. Get a nVidia GTx 560 Ti if you want to save some of that money, or if you are gonna throw it all in a GTX580 or a 590 if you can get a good deal, if you went with the gigabyte board get the gigabyte gfx card for maximum compatibility!