First, any 10 Base-T hubs should be replaced with 100 Base-T switches and 10 Base-T Ethernet cards in computers should be replaced with 100 Base-T cards.  This is about a 100x improvement in network performance for under $50 per computer.

Second, replace any 100 Base-T network hubs with 100 Base-T network switches; you could double the potential available network bandwidth.  The cost of network switches has dropped so much in the past 2 years, that this is a very inexpensive way to boost network performance.

Third, since the ONS servers support Gigabit Ethernet, you might consider buying a network switch with at least one Gigabit port. An 8-port 100 Base-T switch with a Gigabit uplink from 3Com costs less than $100. A switch with 8 Gigabit ports runs about $150.