i have an SAPPHIRE Radeon HD 7850 - 2 GB GDDR5 - PCI-Express 3.0 (11200-07-21G), and would like to know if it is better to crossfire with a same one or buy i new graphic card to have 4gbs. Thank u.
If you want 4GB of video memory you will have to buy a new card. When you run cards in crossfire/SLI you don't double your memory, you will still only have 2GB.
Get a GTX 970 4gb if you have the $$$. I would only recommend a 2nd HD 7850 if you could get one for really cheap and you have a large enough PSU to run both cards in CF. Or for a bit cheaper go with the R9 290 4gb.