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. http://www.newegg.com/Product/Product.aspx?Item=N82E16817371059
Something like that should be able to run 2 4870's in CF.