I know of the HDD and the Graphics Cards that you can upgrade your Mac with. However...and correct me if I am wrong, Macs are different in the sense that the amount of hardware that could be compatible with the architecture is very limited. Not many manufacturers invest into producing this type of hardware. As a result, although it would be possible, there is not much point of upgrading a mac (in my opinion of course). Aterall Mac holds a 3% share in all computers...which I consider "nothing". I am not therefore surprised in the lack of investment...and the subsequent low adoption rate of Macs.