I think it will be a while before games actually start utilizing DX 11.
It seems you want to be playing WoW, in that case I'd go with the 4850, which will easily pump out 40+ frames on maximum settings depending on cpu.
It's very cheap (I found one for $95). It will tie you over for a long time until the new cards come out and drop in price.
You can never future proof a computer, whatever GPU you buy now will be outdated within a year, so hold out as long as you possibly can until you actually get a game that requires a beefy GPU, THEN go for the powerful card, buying one now when you can wait 6 months and get it for half price is a waste.
Less focus on future proof more focus on now, look at the games you will be playing within the forseeable future, if they aren't graphics intensive, don't buy it. the 4850 is enough for any game currently out there at reasonable settings.
Also you said you want to play Battlefield 3 at decent settings which comes out in 2011? If that's the only PC game you are looking forward to, you can buy the $95 4850 today to tie you over, and when that game comes out you can buy the most powerful card currently available on the market as of right now for even cheaper than that.
