Just to add my $0.03 (weak US dollar, you know,) my general theory is that the OS that shipped with the computer is always okay; otherwise you need the disc.
So a dual 450 MHz G4 is a "Mysitc" Power Mac G4 (Gigabit Ethernet).
That came with Mac OS 9.0.4 only. (It was discontinued before OS X came out.) Although there is a slight possibility, if it was the 'server' model, that it came with the pre-Aqua (aka Rhapsody) Mac OS X Server 1.x.
It is officially capable of running 10.4, and with some hacking, can be made to run 10.5. If it has a newer-than-stock video card (Stock is a Rage 128,) then 10.5 may be usable. Otherwise, I wouldn't even try. (10.5 is barely usable on my single-proc 466 MHz with a GeForce 2MX, but it is usable. With a Rage 128, it's not really usable.)
So a dual 450 MHz G4 is a "Mysitc" Power Mac G4 (Gigabit Ethernet).
That came with Mac OS 9.0.4 only. (It was discontinued before OS X came out.) Although there is a slight possibility, if it was the 'server' model, that it came with the pre-Aqua (aka Rhapsody) Mac OS X Server 1.x.
It is officially capable of running 10.4, and with some hacking, can be made to run 10.5. If it has a newer-than-stock video card (Stock is a Rage 128,) then 10.5 may be usable. Otherwise, I wouldn't even try. (10.5 is barely usable on my single-proc 466 MHz with a GeForce 2MX, but it is usable. With a Rage 128, it's not really usable.)