The xbox is basically a p3 733mghz, 64mb of shared memory, and a geforce 3. It runs linux 'ok' but the resolution is limited to 640x480 and the file system/partitioning for linux is still quirky.
I think the fan may be proprietary, its oddly shaped to snap into the case and does not look like its 70 or 80mm. The gpu fan is 40mm if you have an older xbox. The newer ones do not have the gpu fan anymore.
If you plan to get an xbox to fit into a pc case I would suggest the v1.2 one since it has the standard atx psu 12pin plug on the motherboard.
-efini