|"ok, i knew that"|
I don't think you did, otherwise you never would have asked about using an HD 4670 in SLi.
"i got a friend who is selling a nvidia 8800GTS, and it should be the same series that 8500gt is in wich means they should be SLI compatable right?"
No, the low end 8500GT uses the G86 GPU, the higher end 8800GTS uses the G92 GPU. They can NOT be run together.
Do you even have a power supply that's capable of handling two gaming cards? And I'm not asking about wattage, I'm asking about amperage.