Keep the monitor, downgrade the card. Both the HD5850 and the HD5870 can overclock to about 1ghz(on cards that allow voltage changes). At those speeds the HD5870 is only 5-10% better which isn't nearly worth the extra $100 imo. That said even at stock speeds the HD5850 is very good for 1920x1080. You'll even be able to max out Crysis with 2xAA and play smoothly.
This would be a good choice; http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Here's a review; http://www.legitreviews.com/article/1273/1/
As for crossfiring the HD5970 is actually 2 (underclocked) HD5870s on one card. You can crossfire them with the only limitation being that the HD5870 would underclock itself to match the clock speeds of the HD5970(irrelevant if you are overclocking.)
Likewise you can crossfire an HD5870 with an HD5850 but the more powerful card (HD5870) will essentially lower it's specifications to match the weaker card so it will be the same as crossfiring two HD5850s.