From what I understand, it kinda depends on the use case. I'm running an Nvidia 650 GTX and it's serviceable up to 1080p in most games at fairly high settings, for (when I bought it right after release) $119.99, and in that price range it's an excellent card. Handles 3 screens without Displayport as well, unlike AMD, as long as you get a non-reference card with the outputs to do it.
I've actually kind of lost touch with AMD graphics chips in the last year or two, so I can't really speak for them. Reviews are generally helpful. From what I've seen, Nvidia's 660 at around $220 seems to be a pretty good balance of price and performance, giving you the power to run more or less any game maxed on a single screen, with fairly good multi-screen support. If you don't mind being a generation behind, the 560 ti's are on sale for significant discounts right now, and with that they are pretty good value.