The only ATi cards that support DX11 (and any of its features) are the HD5xxx series.
If you crossfire two HD4850's, you will receive roughly a 60-80% boost compared to a single HD4850. Some games do not scale well in crossfire and might have a considerably smaller percentage boost. The article here has some benchmarks comparing a single HD4850 to two Crossfired HD4850s:
No it won't support DX11 at all. It will run up to DX10.1.
If you run 2 of them on a motherboard capable of crossfire, yes you will use both of them and increase performance, though it will not be double the performance of one as the scaling and communication etc. is not perfect. Lately crossfire has been showing around 80% increase in performance, though each game will react somewhat differently.
The 4800 series has a tessellation unit that was never used. I don't know if it is technically DX11 compatible or not, but a modified version of it was carried over to the 5000 series cards. It's also missing a few instructions that would qualify it to be a DX11 card. I'm sure if you were to go over the boring developer tech documents yourself you would find out which ones.
Thanks guyz but i read some where that dirextx 10.1 cards will support dx 11 but not all the features is it true
What you read refers to the fact that DX11 is a superset of DX10/10.1 which basically means that in a fully coded DX11 game the DX10.1 card will run what it can of the DX11 code which in turn really means that its a DX01.1 card and cant run DX11 features.
They may i think (it was a while ago we discused this)be able to do some of the direct compute stuff but for the purposes of what you are asking and in laymans terms then no they cant run any DX11 features that would actually matter to your gaming experiance.
Many DX11 games will run on it, but will not be running any DX11 specific features. You can't turn on tessellation or any of those DX11 advantages, so basically you will be running a DX10.1 game at that point, so it still comes down to NO the 4850 IS NOT DX11.