How can a single HD 5970 4GB beat two HD 5970s at 2GB? That one 5970 wont have the performance to beat two 5970s. My 5970s have a total of 4GB. So in that point the memory is equal as well. And how the heck will two 5850/5870s 2GBs beat two 5970 2GBs??
Dude.. you spent like $1200 in graphics cards and you don't know your graphics system only has 1 GB of available memory?
The HD5970 has 2GB, and its internal Crossfire requires that each GPU gets its own memory. So a 2GB HD5970 will only load 1 GB of graphics data. The same will happen if you have 2*HD5970, because the second card will do exactly the same.
So a single HD5850 2GB will have the ability to load twice as much graphics data (textures, shaders, geometry, etc) as your 2*HD5970 2GB setup.
And when you fill the graphics memory completely, the graphics system will have to load the data from the PCI-Express bus, which is 20x slower and has a lot more latency. Good luck trying to max out late 2010 games with 1GB of memory..
So having 2*HD5970 is.. well, it's the same processing-power/available-memory proportion as having a HD5870 with 256MB GDDR5. And that sounds really ridiculous.