There is no room (they expect you to expand everything EXTERNAL so welcome to the Mac Mini Pro).
The GPU will not be upgradeable unless you use Thunderbolt (which can't handle anything fast so no, you can't upgrade it).
In short, this is not a Mac Pro. It's a POS that looks like a trash can from a Casino. Expansion is dead. They include 6 Thunderbolt ports (which no one uses), only 4 USB ports (not even certain they are USB3, although one would assume so). Heck, my Mac Mini has 4 USB ports....
They include zero Firewire ports (so much for Pro Audio without an adapter).
They include zero eSata ports (still no
professional in "Pro" I guess; they forget people out there use and need these things whether they think they're the future, past or just crap it doesn't matter; they are used by Pros and Apple doesn't include them)
They include zero free drive bays
They include zero expansion ports (forget all true Pro cards made for the Mac Pro; they expect them all to make new Thunderbolt devices instead, I guess or you to buy some external PCI box expansion to take up all the room they claim you save.
All in all, it's truly disappointing to see Apple sell GIMMICK instead of an actual professional product. Based on the responses, it's pretty obvious that is what Apple fans respond to these days. Throw in the absolutely HORRID looking iOS7 interface shown and I think you can expect the next version of OSX to look just plain GOD AWFUL.