win98 clients are already maximized. For some reason though 2000 pro clients were not maximized. I had a client that opened a directory with over 30,000 files in it. The win98 client opened it in about 5 seconds, the 2000pro client took about 90 seconds. Once I adjusted the MTU's, it went down to 5 seconds. If the MTU is not maximized, it can have a major affect in speed. Especially the inital file open speed. If your server is at 80% for one client, MTU's would be my first suggestion.

A gigabit NIC and switch could treat the symptoms, but I suspect the issue is deeper. Are you 100% 100bt, or do you have mixed NICs. Do you have switches or Hubs?