Two 5770's in CFX actually outpace the 5870 by a slim to medium margin depending on game. Over $100 in savings, in my opinion, is worth it.
The only problem you'll have is that in the distant future (because it will be fairly distant) is your 5770's will have reached their maximum usefulness for gaming. Getting one 5870 now will allow the option of getting a 2nd one in the future, but by that time you may end up getting an entirely new rig.
Slightly better performance
Lower power/heat consumption (I think, if anyone wants to correct me if I'm wrong, please do with your source.)
Crossfire is so kewl with a k
Will end your upgrade capacity
Two 5870's blow two 5770's out of the water
May run into CFX issues, but this is generally easy to remedy.
I have two 5770's myself and haven't seen a problem in gaming yet. If you want to game at your full potential now without care for the future, the choice is obvious.
Sadly I don't even know what resolution. The first thing I bought when getting the rig together was the monitor so that may have been a huge mistake. I got the LG M237WD-PM 23" monitor from Fry's. Say's resolution is 1920x1080 (is that bad??)
I would love not to mess with crossfire but want to be future proof somewhat since this rig is fairly high end.