Mike
I aint no server guru, but here is my 2 cents.
A lot of servers Ive noticed tend to be older computers that are no longer cutting it as workstations, I think your G4 400 would suffice in this dept, It should also be able to run 10.3 - server edition - if it has enough RAM. You can always replace the Hard Drive with a larger capacity one
Keep your G5 for a workstation.
Wait until after Macworld expo on the 6th of January 2004.
There are rumours of new faster G5's and a new G5 based XServe apple dedicated server machine.
www.apple.com/xserve
One other thing I'd do is upgrade your macs to 10.3 it is well worth the investment, even on G3 based computers everthing is far snappier . Hope this helps