In my laptop I have a nVidia GeForce 8400m GS 256MB. For my old desktop I'm purchasing a PCI nVidia 8400 GS 512MB. The only difference is my laptop one has an "M" which I'm guessing is for mobile. My question is, can I expect the same performance for the PCI one I am purchasing for my desktop since its the same card?
Your desktop will actually be faster. By structure and engineering a Desktop with the same ratings/specs as a Laptop will be much faster, that is to say if everything is running correctly and your computers are cleaned out.