i have one w/ a G4 in it. they are very easy to OC, i would suggest swapping the 400mhz cpu and pci cards in from your dead g3, and then OCing the cpu to 450mhz (i added an extra fan in mine to help w/ cooling, it fit in a pci slot).
here is the over-clock guide, it is for a Yikes G4, but the only difference between them and a B&W is that the yikes has a G4 cpu (i used this guide for mine):
http://www.xlr8yourmac.com/G4ZONE/G4YIKESOC/
also, as an interesting note, i believe that one should be able to simply take the G4 zif from their Yikes and pop it into the B&W G3, they have the same or almost the same motherboard.
defiantly put more ram in it, the maximum is one gb, and with one gb it should be a great sever
BTW, what sever software are you using?