i'd say you are better of buying a new card you'll get DX11
what is your system specs, budget and monitor resolution?
A gtx 285 is faster than you think and not all Fermi cards measure up to be an upgrade over older cards. It will take a gtx 560 ti just to break even and see a small gain. If op really wants to see a real upgrade by going Fermi then I suggest a gtx 570 or he could just go on ebay and try to get a second hand gtx 285 on the cheap then sli.
If you want to go ati then there are some decent choices out there like the 6950.