Judging by the GPU Hierarchy Chart -xfired they are close to AMD 6970 or GTX570 (which i think can run pretty demanding games at the given resolution choices).
Would you guys recommend using one or two cards?
Which screen choice?
Would you even build a system for this? (What would be the cheapest possible solution build to maximum the cards potential? (I would not need OS/GPU/Monitor)
AMD has dropped driver support for the 4000 series and before. You will likely have problems running any new games that come out. If you go with a Crossfire setup, those problems will only be worse as Crossfire specifically relies upon game profiles to run adequately. If the games you want to play are currently supported with Crossfire profiles, then you should be fine, but don't expect much for the future.
I would recommend getting something like an i3-2100 or similar, as it would be cheap, but is still a very fast chip. You could also use a 965BE, but the i3 is faster.
You could possibly make it into a media center PC and use it on the 46" TV, Just add a Blu-Ray drive and you're set.
You could plug it into the 25" monitor in crossfire and get respectable performance, but you already have a gaming computer.
You could keep it as a spare just in case something on your current computer dies. Or give it to a friend or sell it or whatever.
Crossfire solutions generally produce more heat though and use more power, so if you are paying your own bill then you might want to keep that in mind. Also sometimes there are driver issues and instability along with microstuttering, but its not like its ever a huge problem.