A single 270X is about $170 so two is about $340. Two 270X's will out perform a single 280x, 770 or even 290 (all around $340), but Crossfire and SLI are prone to compatibility issues, so while you will get better performance when it works properly, you are also opening yourself to risk of them playing up a bit. For this reason most people would still opt for the 280x or 770 in their initial builds if they can afford it. However, if you can only afford a single 270x at the moment, and were thinking of adding a second one later than that is still a good idea, as on the occasions it doesn't work, you are used to playing on a single 270X, and when it does work you will have a huge graphics bump.