280X is a rebranded from 7970. 290/290X is a totally new designed cards. As for how close between the two in rendering speed. Thats may be different from machine to machine because of different setup etc each person have. Go with the new designed card; 290 is a better choice, cost a little more. But definitely worth it.
I had a lot of (unsolved) problems with the 280X. Could not stop my 3 monitors flashing when I clicked from screen to screen. Exchanged it for an Nvidia equivalent.