I'm sure everyone here will agree when I say don't go AMD for a gaming build over $500 or so, Intel's Sandy Bridge architecture is just superior for games.
SLI/Crossfire is multiple graphics cards. This is only really worth it for super high budgets or super high resolutions in my opinion. The reason being that despite the good value it will normally run very hot and very loud and consume far more power.
@OP - You see the pattern developing here... If you are open to CF / SLI then the Z68 motherboard is the route to go with multiple high speed PCIe slots. If you want the simplicity of a single graphics card, then an H61 / H67 motherboard is a good opiton. Don't put too much stock in the PCIe 2.0 vs PCIe 3.0 debate. Either will perform the exact same... Also, the i5-2400 and i5-2500 will game the same. The 100MHz speed difference won't be noticed so save a little cash and go with the i5-2400.
if your not overclocking i agree go with the 2400. i would get a board that will support crossfire because game are gonna get harder to play in the coming years and your going to want to be able to upgrade. either start with a 6870 or a 6950 as these will both do you well and are exceptionally good for the price especially for the price. with that in mind if you dont want to spend more money down the road get a 650 or 750W PSU if you are going to xfire the 6870 or 6950 accordingly in the future. other than that since your not overclocking at all the other parts are up to personal preference and price.
Don't cheap out on the case, it's the part that will last the longest and is holding everything together. I can personally recemmend the coolermaster storm sniper. Tons of room, works with SSDs, and has great build quality. There is a reason it is still $120 on newegg, it's good. Also sale on AMD procs right now. $150 bucks lands you a 3.7ghz quad core processor with $10 off with promo code. Another 80 - 120 for a good mobo.
also the 6950 runs BF3 better than a comparable 560 ti 448 core. should do ultra with 30fps average meaning its playable at ultra but i would drop it down to high since ultra was really only made for enthusiast with dual graphics solutions.