When you crossfire, your motherboard runs two identical graphics cards at the same time in different ports to double the power.
You need to be concerned about two things.
A. That your power supply can hold all this
B. That your CPU fan is good enough that the motherboard won't overheat.
If neither of those are an issue then yeah I would get it. It's a good video card, even better if it has 2GB.
So close...
Deagonx is mostly right about CrossFire, it's just adding more graphics horsepower to your system, and it can in some cases, improve performance 1.5 to 2 times faster than a single card.
The "concerns" are mostly valid -
A) It is true that you need to be sure that your power supply is powerful enough to handle the added load of another video card. The Radeon HD 6790 isn't one of the highest end cards, and as such only needs 500 watts for a single CPU, and 600 or more for dual GPUs (details from AMD's website)
B) The CPU fan has nothing to do with the motherboard overheating in the case of a dual video card situation. True that a good CPU fan is vital to the overall health of a system, if the system is running fine now, there's no reason to suspect that a second video card will cause issues with the CPU fan. You'll want to shift your focus to the case fans, and any additional heat that they have to handle with the addition of another video card.
Fortunately, point "B" will not be a major item of contention, as the 6790 isn't as hot headed as the 6870/6890 or above video cards.
Lastly, memory amount won't matter on a video card setup unless you're running a very high resolution. Most folks think that more RAM = better video card, but only to an extent.
Those points being said, if you're noticing problems in games, or want to simply upgrade a little ahead of what you have now, getting another 6790 to use in CrossFire isn't a bad idea, as long as the price is right. If the price is about the same for say the next generational step up, then it makes more sense to upgrade to the next video card than another one for CrossFire action.
I'm new to this. I try to give my best advice though.