Considering this is just for personal serving, here are a few tips for you:
1 - 10/100 Ethernet is fast enough. unless someone is hitting your server every second requesting big files, your bandwidth is fine.
2 - a B&W G3 is actually going to be bulkier than the iMac, when all things are considered. The iMac is shorter and shallower, but wider.
3 - The iMac is quieter, by far, and will support OS X.
4 - If you can find a cheap Cube, go that route. The upgradeability is better than most people will admit for the file service you are talking about. You can always attach a small-footprint 250 GB Firewire drive if you need more HDD. You just need one with a CD (no burner), 10/100, replace the HDD with a monster and max the RAM. Plus, they are super-quiet. However, like the G3 B&W, you will need a monitor for initial setup.
I use a G3 iMac for file service and would only replace it with a Cube, for space considerations.
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