Recently I had to upgrade my power supply because the fan in my old one somehow decided to start rubbing against the side of the power supply and soon shatter, so I had to go ahead and buy a new, better, power supply.
Anyways, the deal is that I'm a solid MW2, Dirt 2, and Bad Company 2 player.. 2 out of 3 of my favorite games have DX11 support, so I am going to upgrade to one of the new ATI cards, my only hold back is I am not sure whether it is better for price/performance to get two 5770's, or a 5850.
I have two monitors, 25" and 22", they run at 1920x1080 and 1680x1050. I only play on one monitor (obvious) and use my secondary for multitasking. I'm powering this with an AMD 940 Phenom II
2xHD5770 in Crossfire will perform better than a single HD5850. http://www.legitreviews.com/article/1105/5/
I can not find reviews that contains your favorite games but I think this benchmark will be about the same.