Even on a lower resolution display, a 7870 can be great. Just pump up the AA to make up for the low resolution. Of course, upgrade the monitor if you want to, but it's not like the 7870 would be wasted there unless you play games that simply can't be made very intensive on the graphics and other than a few lower end games such as Diablo 3, I can't think of many such games that aren't pretty old.
I also wouldn't say that the 7870 eats the 660 for lunch. I think that they compete very well, but that 7870 is one that I found earlier myself and I can't find any 660 less than like $30 more expensive right now and I'd recommend it. Now if it was something like a 660 Ti versus 7950, then I'd say to avoid the 660 Ti like the plague, but I like how well the 660 competes with the 7870. I consider the 660 to be the best value of the GTX 6xx generation right now.