I don't have MachDB up and running yet, but I've been working on rolling my own and figured it might be worthwhile to give what aid I can to MachDB.
A few fields I'm currently collecting in my own db (really awful 2AM hack with a Python collector -> XML -> php parser -> MySQL):
-Memory Type, FormFactor and Speed (so I don't need to pull mfr specs when ordering more)
-NICs - encap, highest speed/standard (so I can see which legacy boxes are still running 100baseT/full, what has gig capability, and which boxes have fiber)
My take on PCI slots:
I track the slot type (length, type (PCI[-X|-E]), identifier (usually something like "PCI Express x16", etc.)) and whether it's populated or not - not so much for figuring out what's installed, but for figuring out which hosts can accept card X for device Y.
Hey! I have started working on MachDB again and moved all bugs to http://www.mybrainhurts.com/redmine/projects/machdb/ I have moved this over and you can now track it there going forward. Thanks for the submission BTW. :)