But I am getting a pci-e 3.0 motherboard so both are going to run at x16 so they shouldn't bottle nick should they?
When it comes to screen resolution the most important thing is how much onboard memory is on each card. Even though a crossfired 5970 and 5870 would have tons of shader power, they would struggle to power 6 screens because they would simply run out of memory, or frame buffer. A lot of the shader power would be wasted because without the memory to hold the information on all 6 screens, the frame rate would just crash. This is also why Nvidia cards struggle at the top resolutions unless they have 3 Gb per card. Even 1.5 Gb per card isn't enough in some games at very high resolution.
With 6 screens you definitely need to get 2 Gb per card trust me.