Check the server admin network traffic graph and see what kind of average traffic you are looking at. Also, you can run Activity Monitor to get more concise traffic readouts.
My suspicion is that you are likely using a 100Base-T hub/switch to connect this machines. If so, you aren't likely to even see 10MB/sec of network traffic. I assume you are at least using a PowerMac G4 as your server and therefore you have gigabit ethernet on the server. If you use a gigabit ethernet switch instead of a 100Base-T switch, then you should get better throughput when multiple clients are connecting simultaneously--even if the clients are only 100Base-T. In addition, at this point, your hard drive is likely going to be the bottleneck. If so, you may want to look into RAID (the SyncRAID cards over at
www.macgurus.com look very interesting) for speed and redundancy.