Personally I feel that the 550ti graphics solution you have chosen is rather sub-optimal for the amount of money they will cost you. It is true that the SLI performance is better in most cases than a similarly priced single card solution however, if your intent is to run this game a well as possible, I would be somewhat wary of a dual gpu setup because the driver optimizations may be limited, which could cause issues with microstuttering and the like.
This being said, you seem partial to nvidia, so I would recommend a single 560ti graphics card over the 550ti x2. This will give you future upgradability and allow your system to be further future-proofed, as you would soon find the 550ti sli isn't powerful enough to max out many of the games on the market now whilst achieving solid framerates at 1080p. Personally, I find that the amd7850 is a better buy for the money in every regard as compared to the 560ti, though if your goal is physx support and the reliability of nvidia graphics processors, you should probably go with your gut there.
For anyone who says that FX > SB for the money, it just isn't true. My PC build right now consists of a FX-8120 OC to 3.8ghz coreclock, and it roughly compares to my old SB-2500k at stock. Even for media encoding and applications the 2500k would out-perform my new zambezi in some areas due to the optimizations intel integrated into their framework. Bottom line: an overclocked 8120 can compete with a stock 2500k, and likewise, an overclocked 8150 can compete with a 2600k, but we are comparing OC vs. STOCK. OC the SB and it will blow the FX out of the water at a slightly higher price-point. In fact, the old 1090t had better gaming performance in some benchmarks than the FX 8core series.
It's not all that black and white though. Considering the framework that the FX series processors are based upon, and the under-utilization of its cores in most applications, there is room for future improvement within the guise of optimizations geared toward multi-threaded performance in games, and better coding framework within media applications. This, alongside the brutal OC performance of the chip had me leaning toward it when I built my 7850 CF gaming build.
This all being said about amd cpu performance, the amd 7000-series graphics solutions are rather remarkable. If you are hard pushed toward nvidia, consider waiting for the rest of the 600-series lineup to release, as I feel that the playing field will be levelled out better, and the price:performace ratio will only get better. As it currently stands, AMD wins every price:performance battle beside the 7970vs680. Kepler drops drawer and unloads on the Tahiti just like it had done on the Fermi not-too-long ago.