Like ray said, it's not really going to make a difference. Both cards a graphic powerhouses and will run any game you throw at them, especially when using 2 of the same. I am planning to use two 9800GT (512MB) cards in SLI myself; I also play those games you listed. You can be sure to see a huge improvement in fps and in image quality.
I play the same Source games you play but on a 8800GT. I actually had an 8800GT SLI configuration at one time and didn't see any real improvement from the second card. I average over a 100FPS on a single card and saw no reason to continue to waste two video cards on one computer. I'd just keep what you already have.
And like mailpup said, you'll need a new nVidia based motherboard to enable SLI with two new nVidia based video cards, so things may be a little more expensive than they originally appeared and you may not see several hundred dollars worth of improvement after all your effort.
why not use ati cards as a gamer,designer or photo editor?
some games offer you better frames on nVidia,others like ati better....
but with either ones(or in that case two) you get to play all games out today
with good image quality and good frames per second.