ive got a asus 6870 and powercolour 6870. do both these cards perform the same or are there some differences. Also, would these two cards work well in crossfire.
Normally if both graphics cards have the same gpu and support crossfire they will work together easily but there's no way to know for sure unless you see someone else running those two cards together. Amd say it should work http://sites.amd.com/PublishingImages/Public/Graphic_Illustrations/WebBannerJPEG/AMD_CrossfireX_Chart_1618W.jpg and Amd are a lot better than nvidia at getting different cards to run crossfire together but normally they don't scale as well as nvidia's cards do.
I can't ensure they will work, but I would expect they would, they're equal to something like gtx 560ti or gtx 650ti boost. So when you run them in crossfire, they will defiantly be able to play crysis 3 on medium or do whatever you need them for.