If I am building a computer for someone and I use a system builder disc with Windows 7 Ultimate, can I re-install the OS with it if their computer crashes? (re-format, re-install)

I don't see why not. AFAIK the system builder disc's only real limitation is that you can't transfer the license to another computer

So basically, if it is the same motherboard, it is safe to re-install?

The WGA activation considers other parts as well when determining the uniqueness of a machine assembly.

There are pre-determined weight factors for each type of part, and the motherboard does have a lot of influence there. However, it is not by far the only part that the activation process looks for. Processor and memory, for example, are also high on the list. I don't know if the actual factors are public knowledge or not, so I don't mention the numbers here.

If an automated Windows activation fails, there is always the option to call Microsoft directly via the number the activation program gives you. If you can explain your actions to them so as to assure them you're on legitimate business, you can get a new product key that will activate the machine. Note that they do check what your license actually allows, so you can't really install an SB OEM key to multiple machines.

The point is, if you're legit, things are going to work just fine.

I've had to call MS twice in the past to get a key (for XP), and they would always just give me one with no questions asked, so both times I wonder what triggered the need to call them in the first place!! :P