The 670 is a universally good choice, especially at 1080p. If you're committed to that monitor resolution, you can't beat it.
If you want to go cheaper than $400, let me suggest the twin frozr 7950 at $310 post-rebate: http://www.newegg.com/Product/Product.aspx?Item=N82E16814127667
It's a really good deal right now (hard to believe it's only a few bucks more than the 7870). The thing overclocks like crazy and has 3GB of VRAM, which will help you if you push beyond 1080p.