maybe a gtx 560 on the cheap:
you would have a lot of power for that resolution given that you have ample cpu
how about this one:http://www.newegg.com/Product/Product.aspx?Item=N82E16814130660
evga and a lifetime warranty
or pny has a refurb with optional 1 year warranty for 150:http://www.newegg.com/Product/Product.aspx?Item=N82E16814133430
in the amd arena id go for these:http://www.newegg.com/Product/Product.aspx?Item=N82E16814102948
^that might be the best deal out there on the egg for a 6870
the 7770 is just not a good deal if you can get a 6870 or gtx 560 for liek 10 dollars more... not worth it unless you are running under strict power requirements which is sounds you get the right idea of getting a solid 5-600w unit.
with the cards that I rec. above at least you will get playable frame rates if/when you decide to upgrade your monitor to 1080p with something like a 6770 or 5770 it will still game at 1080 but not nearly as good as the gtx 560/6870
..and since the 7k series just got released, you can get a good deal on a previous gen card that is more powerful for the money in that segment.
Now if you wanted to move up to a 7850 then id say go with the 7 series but for your budget and performance needs Id say that the 6870 or gtx 560 would suit your needs quite well