6800(AGP ONLY) has a 50/50 chance of being unlocked via software to a 6800gt core(though it wont have the same amount of memory so it wont be as fast as a 6800gt)
6600gt really starts to suffer when AA is turned on or higher resolutions due to half the memory bandwith(256bit v.s. 128bit) and half the pixel pipes/vertex shaders(12pp/5vs for 6800, 16pp/6vs for unlocked 6800, 8pp/3vs for 6600gt)
edit: never, ever SLI an 6600gt. for the same price you can easily pick up an 6800gt or maybe even a 6800ultra when it is on sale. the 6800gt itself will blow away an 6600gt when you use higher resolutions/aa.(12x10 or maybe 2xaa)
after all, why would you sli when you can get a faster card for our needs that uses lwess power, less noise and dosent take up all that space and works at the 6600gt sli speed in non sli enabled games?