Then a single HD 5770 is all you'll need at that resolution. IF you want better longevity, adding a second 5770 would be a good option, though you'll be able to max out every game with a single 5770 at that resolution.
5770CF is NOT faster than 5870, it's falling few percent behind, and i am always for the system with single GPU rather than dual if the price is equal, but for this resolution 5770 should be fine and adding another card wouldn't make much of difference.
I just replaced two 5770's with a single 6950. The two 5770's were awsome. It is very cost effective. It sometimes had a couple of issues with heat with poor spacing, but it ran every game very well. With the cost of a 5770 right now it's a great option for you. With that said my 6950 is better, but like you said an expensiver single card is not an option for you.