Well at $500, the GTX 780 is a good deal but when the R9-290 is released, it will most likely piledrive the GTX 780's price even further into the ground. I would wait and see if I were you. If you must buy it this minute, then it depends on what resolution you use. If you use 1920x1080, then the GTX 780 is the way to go because it more or less matches the R9-290X at that resolution. If you game at higher resolutions, the R9-290X reigns supreme because of its superior memory controller. I would wait if I were you because, just as I feel really bad for people who bought the GTX 780 within the past month, you don't want to get caught spending $500 if a) The GTX 780 drops another $50 or more in price or b) The R9-290X is priced so low that the GTX 780 cannot hope to match it.
The R9-290 should be released fairly soon but as I said, if you can't wait, then get the GTX 780. At $500, it's a really good deal. Here's one to consider:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127740