You should really consider the R9 290 as well, if you want to save some money. It gives a bit less performance than GTX 780 whereas the 290X gives slightly more, but it costs a lot less. Make sure you also get a model that's cooled effectively by multiple fans, you don't want a stock card. Also, AMD GPUs can use Mantle, which gives a pretty substantial performance increase.
I would recommend this card:http://www.newegg.com/Product/Product.aspx?Item=N82E16814125500&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-na-_-na-_-na&cm_sp=&AID=10446076&PID=3938566&SID=
It's only $400, and it's factory overclocked with triple fans and an effective heatsink. It should give you excellent performance, comparable to a GTX 780 that would cost you at...