You are good with that setup but I'll recommend you get another 4GB stick of ram(same frequency and latency)
You should change your motherboard to a better board like ASRock Z77 Extreme 4
You need to have a power supply of 700watts run this setup but you can get a 800watts one just to be futureproof.
Two HD 7970 GHz Edition cards in crossfire is better than two 680 at SLI http://www.anandtech.com/bench/Product/585?vs=588 (the link here uses a normal hd 7970 not a GHz Edition card which is faster)
Your processor is ok but consider to upgrade whenever you can.
Honestly I think two 680 cards is too much for your specs with one 1080p monitor.
2 680 in sli would be an overkill for 1080p...consider upgrading your monitor and the mainboard.
the mainboard is infact must be upgraded to a better one. Look for 1 pci-e 3 x16 slot and a x8 slot atleast.that will give you the full benefit of the 680s in sli.
A single GTX 670 or HD 7950 level card is what you are looking for. Even that will need you to either overclock the CPU or upgrade the CPU/motherboard.
^ These guys don't seem to understand you have an LGA 1156 CPU that will not work in the LGA 1155 boards they are recommending. Since you are considering over $1000 just for 2 x GTX 680s a new CPU and board may be something you are interested in.
You will also need a new power supply even for a single GTX 670/HD 7950. I sure don't know of any quality 460w models.
So, to just add a single card you need the card and a power supply and to overclock the CPU at least if you want the most out of that card.
For 2 x high end cards it's new system time.
8GB of RAM would not hurt but for just a gaming machine 4GB is sufficient most of the time.
first of all i would not advice going for an sli setup for a 1080p display at all.
all you really need at that resolution to max games out is a 670. also you need a better psu even for a 670 as it most likely does not have the required connectors. so my choice is a SINGLE 670
I agree. All you really need is a single GTX 670 for 1080p. Spend the extra $650 that you are saving on a 700w power supply (800w if you want to futureproof your system), and a new motherboard and CPU. I suggest a 3570K with an ASRock Z77 Extreme4. With this setup, you should get 60-100 FPS in nearly every game easy.
If you do not upgrade your motherboard and CPU to new ones that support PCI-E X16, you will NEVER be able to fully utilize ANY modern graphics card. They will ALL underperform.