Well we do know for a fact that the 5720 can run the 7900 Go GTX. I have heard the 512mb version as well. I know you can upgrade through a company in Germany (Cyber something, approximately 600 Euros = Ouch).
Now Sager has said something a little different. They would'nt confirm or deny whether or not an upgrade was coming. PCT Luke has been prodding them about this and they are supposedly doing compatibility testing or have done it. I think a 5720 upgrade path from Sager should be coming soon. However, don't hold your breath on the 8000 series card.
I told Sager I wanted them to upgrade me to a 5760 from my 5720, they said while possible obviously, they would not do it at this time.