Dual monitors on my home built

If you are using the monitor for something other than gaming, it does not need to be the same resolution or size. This could save you some money by purchasing a lesser monitor, but if you want things to look uniform there's nothing wrong with getting a duplicate.

To seamlessly switch between your game and web browsing, you need to run the game in Windowed (Full Screen) mode. This makes it so you don't have to alt+tab to move over to your web browser windows. Unfortunately, this mode disables all but the primary graphics card, so crossfiring wouldn't improve your performance. I found this out the hard way when I got 5850's in crossfire.

The good news is that a single 6950 should be plenty to power the kind of games you are playing at 1920x1080. If you want more performance and you don't mind running Full Screen mode and alt+tab'ing out every time you want to do something in your browser, crossfiring isn't a bad option. You don't need an 850w psu for 6950 x2 though, a 750w should be just fine (assuming it's from a reputable brand with good quality like corsair, xfx, etc). Finally, there are 6950 1GB cards available. It doesn't have to match your current one, any will do. Don't spend the extra $ on a 2GB- you won't get any benefit out of the extra memory pairing it with a 1GB card. Even if you did, the two perform IDENTICALLY at your resolution! Save the cash!