You can certainly upgrade the RAM and Harddrive, and maybe eventually the optical drive, when the super-mega drive, or whatever supplants DVD-R comes out, but other than that, well, not much. You're basically limited to the memory, harddrive and optical drive, or whatever you can shove in the pc card slot, but that's about it. The processor is soldered to the motherboard, same with the graphics card.
You could always get a PCI expansion chassis, those are pretty cool, but also pretty expensive. It opens up the option to add pretty much anything you might add to a desktop machine.
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