Currently, I am using MSI H55GD65 motherboard (PCI express 2.0) and want to know if the 7950 would work fluently with it. I would need to upgrade my PSU too. So, how much power would you recommend for the 7950? I am thinking of 500 or 650 watt. I want to max out Battlefield 3, Crysis, batman: arkham city and other games on 1920 by 1080 res. Am I making a good choice? Should I consider any other GPU?
As long as the motherboard has a PCIe x16 slot, it's compatible. 7950 is a good choice though I know many who would disagree and would recommend something else (GTX660, Radeon HD7870, GTX660ti are also good choices). Depending on you CPU, you'll either see huge improvement or very minor if it's bottlenecking the card. For psu, I recommend the Seasonic S12II 520W to start.
I actually have a MSI 7950 Twin Frozr. From my experience I love the card and can play everything maxed out. My only gripe with it is that I was not impressed with the cooling solution and noise produced by the twin frozr fans. I actually went out and bought an aftermarket GPU cooler. So from my experience, I'd recommend the Gigabyte GV-R7950WF3 or the Sapphire 7950 Vapor-X as they are the same price and have much better cooling solutions.
As for performance and compatibility, you won't be disappointed The card will work perfectly fine with your current set up and you can even get the 7950 close to the performance of the 7970 with overclocking.