If you're going to go AMD, then you ought to look into AM3+ boards. The 550ti is underpowered at 1080p, and even if those boards could SLI, the Tom's editors say they microstutter when set up as such. SO, I'd recommend going with an AM3+ mobo, and putting as much money as you can into the GPU, grab a 560(ti) or a 6970 if you can, along with a board that can SLI/CF.
Edit: Meant to mention that I'd go with 4GB of some faster RAM, you can save some money and up your performance. I like these:http://www.newegg.com/Product/Product.aspx?Item=N82E16820231443
Edit 2: I see you didn't mention a resolution, I got 1080p from the last thread I was looking at.