Looks like the first GM of Mavericks was Oct 4th and it was released on Oct 22nd. That being said, I doubt you'll see it before any kind of Apple Event which I expect sometime within the next week or two.
Well it was rumored in this article that they'll switch to AMD graphics which is why I mentioned those chips. I myself would much rather see NVIDIA chips vs AMD ones. I've kinda always been an NVIDIA fan for some reason. I guess the thing is...could Apple fit an NVIDIA 980 or 970 into the enclosure of an 27" iMac and still meet power and cooling requirements.
My guess is they'll have to use something like the AMD Radeon R5, R7, or R9 mobile graphics chipset. I would think they'd use the R5 as Apple never really chooses the absolutely highest possible, even for a BTO. http://www.amd.com/en-us/products/graphics/notebook/r9-m200