I have a crappy Dell E177FPC monitor at the moment. (1280x1024) I know...
Should I get two more and run eyefinity or buy a single 1920x1080 monitor?
EDIT: If you happen to have a Dell E177FPC monitor, I'd like to buy it off you.
Power efficiency wise vs resolution, it's best if you just run a single monitor at 1080p, then keep your existing monitor for something else like an additional display.