Hi,
I think that there is not a huge difference between the S670 and the S775, they have the same core, just that the S775 have a little more of evrything.
So, If you get a good deal on the update you could do that but I'm afraid that you will get a bit disappointed "Was this all?"
So I think it would be a better deal to go for the S970 (or higher, if you have the budget)
Kind regards Marcus