I would suggest that you go with a used 4890. That is what I did.
My reasoning is that there is too little games with support for DX11 atm, and NONE that require DX11. This will almost certainly remain so for at least another year, and even if there will be a somewhat significant amount of DX11 games, most of them will very likely still support DX10.
The reason why I am saying all this, is that in all the time that DX10 was available, I saw only a handful games that truly took advantage of the new tech, the rest, only talk about supporting it, but in reality, when DX9 and 10 modes are compared side by side they look virtually identical. This will very likely be the case with DX11 and when/if this changes mid range DX11 cards will cost around $150. So for $300 I will have an awesome card now, and another very capable one in a year. ; )
I didn't feel like spending $300 for a 5850, so I went with a much cheaper option that will easily rip through any DX10 game, and play them on max settings.