Yes, an HD5770 is the most you should consider on that monitor.
But if you are saying you can afford 2 HD5870s then just get a good 1080p monitor and SLI 2 GTX 460s. That would be the best way to spend your money.
the only good thing about using dual 5850's on thatr size screenis benchmarking!
otherwise what about a screen upgrade while you at it!! thats the only way you can really appreciate two 5850's!! to have two or more cards is nothing, the main purpose is to appreciate them, for what they are worth, and what they can really offer you!!!
Notice how I said in a single card configuration the 5850 would be good while in a dual card configuration 2x 460's @ 1GB would be good? Pay attention!
jyjjy already suggested 2 x 460's, the other section you took my words about :if the HD5850 was overclocked and heating, if you have something to say, try be a bit more original, rather than concuring over and over again! we are here to help people, not to accumulate status points with information we've heard already!!