You can use either 10.3 or 10.4 server... they'll both load with no problem... how well they'll run, though, is debatable on that kind of hardware.
I have 10.4 Server running on a G4 500MHz machine, which is identical to your machine with the exception of the processor. Same bus speed, same internals (basically), same everything. It runs decent, but I would hesitate to go back to using it as my main desktop.