I'd definitely say the 6850, it has DX11 support and is a good deal faster than the 4850. You will see much better performance in just about every game with the 6850. The only time this would not be the case would be if you are playing at an extremely low resolution (1280x1024 or lower).
I would definitely still go with the 6850 at that resolution. You will still see better performance over the 4850, especially in the more demanding titles. The 6850 will also last you a lot longer. The 4850 is already pretty dated, and even at your resolution may not be able to keep up with some of the more demanding titles that are out now, plus whatever the future brings.
Just to clarify resolutions
1440 x 900 = 1,296,000 pixels for the card to process.
1280 x 1024 = 1,310,720 pixels for the card to process which is a higher resolution!
If for nothing else than the HD6850 is newer tech it is still the right choice here!
That is a dual GPU card from about 3 years ago, basically 2 4870s glued together. It is about as powerful as one 6970. However, it is a very power hungry card, if you don't have a really beefy PSU, you won't be able to run it. The HD 4000 series also ran quite hot, and this is exacerbated on dual GPU cards, you will probably have heat issues if you don't have really good airflow in your case.
You also have to watch out for driver issues and microstuttering with that card, as that card runs as 2 4870s in Crossfire, you are more likely to run into problems with a dual GPU setup, not all games support Crossfire properly. Also for your resolution, that card would be extremely overkill, you don't need that much GPU horsepower unless you plan on upgrading your display. Unless you are okay with dealing with these issues, I wouldn't bother with that card. It would be much simpler to go with a single GPU setup.