No performance diffwerence really (Unless you factor in the factory overclock some companies give you). if the 8600GT is anything like the 8800GT all of the PCB is the same. The only difference to the card is the cool picture on the HSF and the companies support.
I say go with eVGA, you can find a factory OC version if you like, you get the step up program, and a great warrantee. they will honor the warrantee even if you overclock/replace the cooling as long as you put the stock HSF back on before RMA. Obviously they wont warrantee it if you say "HEHEH Im retarded this grafix card dosent OC well lets crank the core to 1500 and fry it so i can get a new one to try."
BFG is also a great brand.
You extra money you pay is for the quality of the company not for the card
