Can you crossfire two different graphic cards, same manufacturer but for instance a 7890 with a 7860 or something like that?

Thank you

Yes, but you will not have a good performance boost.

The fastest card will clock down to match the speed of the slowest card if i remember correctly. And the video memory on the second card will not be used. I would advise against it and save a bit more money to put the same card or put a single card more powerfull.

When I had two HD4850, stability was an issue too. Some games would not run with crossfire or had degraded performances (yes lower than with a single card). Maybe it's better these days, i'll let someone else answer that.

Check this.
http://sites.amd.com/PublishingImages/Public/Graphic_Illustrations/WebBannerJPEG/AMD_CrossfireX_Chart_1618W.jpg

thank you

Nice find ! Have some rep.