The largest drive I have used is the IBM TravelStar DJSA220 (20Gb, 2.5" notebook drive NOT a the larger 3.5 desktop drives) The bios is new enough, it should support just about any logical size drive you can find. but only the 2.5" form facter notebook drives will fit inside the GCT case).
The HPNA and ethernet card are mini-pci, you can replace the HPNA with an ethernet mini-PCI or even a Lucent 802.11 wireless min-PCI, I just bought a new Solo 9500 notebook at work that uses the same Realtec mini-PCI card that the GCT uses and sometime ago I sold a GCT ethernet card to a friend who now uses it in his IBM Thinkpad, There is a broad area of compatibility.
The IDE requires a special cable that has evey other line reversed just like the i-opener, Badflash has them.
The memory is 100mhz 144pin SODIMM, these are getting real cheap. I have used a 128mb SODIMM so far with no problem.