What GPU options? Will regular sized GPU's fit in current enclosure?
I would think the Fury Nano would be a good option (maybe the Fury Nano was designed for use in Mac Pros to begin with?). But early reports say the Fury Nano has a terrible coil whine.
no. The new Mac Pro "tube" will only use GPU's provided by Apple in a proprietary daughter board with special cooling and connections, that currently only Apple uses exclusively in the Mac Pro.
And as far as I have seen (haven't checked that recently) there are NO upgrade options after purchase. So you can't opt to purchase the more powerful one at a later date and swap them in yourself (unless maybe you can find them used on ebay or something)