The actual size of your screen doesn't really matter. 19" or 42" doesn't change a thing. What matters is the screen resolution. The 42" i'm assuming is 1080p and I can't imagine your 19" being any lower than 720p. It's the number of pixels on your screen that matters, your 19" might even have the same as the 42".
You should provide more information about what kind of gaming your going to be doing. Just don't waste your money on a 6990 (which is not the best).
If cost isn't an issue you should get the 7970, which is better and actually less than the 6990.
It would be helpful if I knew your other specs such as your CPU, memory, and power supply. Even if you get the best graphics card it won't mean anything unless your CPU is fast enough and you have enough memory.
All truly depends on your resolution what you intend to run at. It's hard to say I'm trying to find sli numbers for metro 2033 It wouldn't really affect Starcraft much the requirements are quite low in my opinion.
This review gives kind of a close up on the 7950 in Crossfire. I know that Metro 2033 is a intense resource hog so anything you throw at it will more then likely eat it up. It looks like it benefits a lot from Crossfire. That being said maybe see if your frames are good and then decide it you want another. If money isn't a option you could check out the second link a 7990 a beast of a card it is.