No need to sell it yet, your card will last well into next year, DX10 cards are supposed to be coming end of year, but if your card is already cutting it at DX9 then why get one, as your only going to be running the same DX9 games, just faster, but if they are already running fast enough on your 7950 as said then theres no point, i would'nt lash out £400+ just to run Vista, and Crysis's DX10 features, probably be only a couple of features at best, and by the time theres more DX10 games around you'll have to lash out £400+ again as your DX10 card will be old hat and not run em well.
Id get a DX10 card when theres actually something out worth getting one for, Vista and Crysis are not that, DX10 games are not going to be flooding out, there might be about 5x by next Christmas, and im probably being to generous there, and how many new cards would have come and gone by then, exactly cards are coming every couple of months, you get all the speedbumped jobs etc....