Normally to gain access to the HD you have to unscrew the PB's keyboard and top case - about ~ 30 screws all up, which actually isn't too bad compared to iBooks and fairly straightforward. The previous owner decided they couldn't be arsed and basically uncrewed about ten screws, then went for it and bent/mangled the aluminium top case off at 90 degrees to get to the HD.
I've since put in an 80GB drive (borrowed from my Pismo) into it, and it's running a fresh install of 10.4.11. No issues, the Geforce 4 seems pretty smooth. One oddity: I bought it knowing it had a "logic board" issue, and I've found the onboard 128MB DDR RAM is faulty and will not detect. Funny thing is it's quite happy to boot still with a 256MB stick put in, just it doesn't chime! I'm going to buy a 1GB stick and see how I go.
I've been really tempted to buy a Netbook recently (eg. MSI Wind for OSX86), but would consider this to be an ever better machine - probably just as fast, onboard firewire, better screen, onboard Superdrive, for about 10 - 15% more size
JB