Not really the 5770 would be the next in line of DX11 cards. The performance increase is slight, maybe 4-10% depending on game. If you are planning on keeping the card for a while I think it is wise to get a DX11 card. Unfortunately, Nvidia does not currently have a DX11 card out.
Just a reminder to take advantage of DX11 hardware and games you will need Windows7 or Windows Vista SP2 (with the video update, it's part of the windows updates that are installed once SP2 is installed).