Seems all the components are available: chipsets, CPUs, GPUs for PCI-E, etc. I expect to see one anytime between the middle of December 2004 through the end of March 2005 at the very latest.

What really makes me drool is the idea of a lighter machine with the LP AMD 64 and the higher end PCI-E removable GPUs: Oh yeah, a Ferrari with an Nvidia Go 6800, heh. Not so sure that's very realistic, but it surely would make me happy.
