If I were you, I'd upgrade for this. But I wouldn't suggest getting the 6850, it's just not the best card to choose for the price.
I'm guessing because you said the 6850 your budget limit for the card is about $150 - $200 right?
If you can possibly afford it I would suggest trying to pick up at 6950 which can be found for under $200 in the US I believe (don't quote me on that for sure because I'm an Aussie but this is what I believe).
Or, you're absolutely bound to find a 5850 for sub $200, perhaps even sub $150 second hand and you should definitely grab a 5850 over a 6850.
However, if you go the 5850 you're limiting yourself a little bit for future endeavours and you said you were planning to upgrade for BF3. If you can push yourself up in budget just a little and get a 6950 then make sure you get a reference design card, you can flash the BIOS and essentially make it at 6970. Then, when BF3 comes out, you can buy another one, Crossfire and have an ABSOLUTE BEAST. This is my recommendation. 1x 6950 flashed to 6970 now, buy another at BF3 and flash it to a 6970 at the time and run in crossfire and have more power than a 6990. Infact, you'd have virtually the second best setup you can get (2x 580s is the only thing better without spending your life savings on a tri SLI 580 setup).