As pointed out by others, the list is huge, and the quickest way to do it is to start somewhere. Pick one, randomly or the one you are using, and visit your manufacture website, select your motherboard model, then click specification, it should list all its attributes. Record all, then build on that.
An example ..
- BIOS (make, model, version)
- Form Factor
- CPU support (socket)
- Front Side Bus
- Memory (type, slots, max)
- On Board
- Storage Interface (ide, sata)
- Expansion Slots (pci, agp, pci express)
- USB (version, ports)
- Connectors (serial, parallel, ps/2)
- Extras (for anything else that is unique to a specific motherboard)
Start with motherboards you are most likely to work with in your database. Not only this will help you start building your database quickly, but will make it more accurate, as you will not add an attribute you don't need, or forget to include one you need.