All Digital Audio G4s accept dual processor upgrades.
However there is no such thing as a Digital Audio 400MHz. That would be the Sawtooth, or Gigabit Ethernet. Some earlier versions of those do not accept dualies. You need to check with the vendor.
Digital Audio originally shipped in 466, 533, 667 and 733; later in higher speeds.
OWC is good; as for the processor upgrade you might want to also check on something like a Sonnet 1.2GHz single.