I have a 4870 1gig card and someone is selling me a 4850 512mb for 50bucks will it work better with just my 4870 since i know that i will clock the card down when i pair it to a 4850. Is this a good deal for me since I wont go dx11 maybe until march.
you are correct in that your 4870 will downclock to a 4850. 4850 crossfire is like having a 5850 in performance and all for just $50? seems worth it to me. But only for a month or two? maybe consider holding back on getting DX11 then. Check to make sure you can even run crossfire and that your PSU can handle the load (550w).
For 50 bucks, that's a swell deal. Plus, you could likely sell it for the same price, or even more later on if you go through a hardware forum. There's always people looking for another 4850 to Crossfire.