The % performance increase from 8600GT => HD 4670 will be significant.
I hate it when people miss the difference between 8600GT and GTS - there is a huge difference in performance. But never the less - nice graphs; they are very self-explanatory.
Neitherway, I'm neither going with a 4670 or a 4770 - depending on prices. I will also be getting a new PSU so I can use my old PSU to build a media center - my older PC is P3 with 4GB DDR2 and a 2400 HD Radeon AGP with a fried PSU - it's outdated but I'm sure it can handle being connected to my 50 inch TV to watch shows in .AVI format.