I just purchased a Z580 which came with the i7 3520M, 500GB hard drive, and 4GB of RAM. I can upgrade the RAM and HDD just fine, but I want to upgrade the GPU as well. It came with just the Intel 4000HD graphics. The built-in GPU is all right, but I'd like to install a Nvidia GT640M. I assume that the motherboards are the same across all the Z580s, so am I correct in thinking that there should be a slot open for a secondary GPU in this thing? And if there is a slot, I am having trouble finding a GPU chip to buy.
There are no slots for graphics card in the Z580. Those Z580s with the GT 640M have the GPU chip soldered to the motherboard. Those Z580s without the dedicated graphics chip do not have a slot where you can solder the GPU.
Also, there is no GT 640M that is packaged as an MXM card, and only high end gaming laptops (GTX 670M or higher) have MXM slots in order to upgrade their graphics card.