Basic measures of a healthy network...

Using switches can be a handy way to increase speed. I don't know any solid guidelines for number of collisions, etc., but I would say on a hub that displays the activity, if you're running over 50% all the time, might be time to do some upgrading. Breaking the network into segments might help some if you've got everything running off the same hub(s).

The number of collisions that you have and still the server is up, that is how many you can have. a network administators main job is to keep the network up. there are two types or NetAdmins. active and reactive. the first is aware of everything that goes on in the server and fixes before broken, ie more ram, more hd space. ect. the second fixes things as they brake...
to speed up the network, the first thing to do is chech you ethernet card. If it is a 10/100 card and your cables are rated for only 10bps you are going to get 10 bps. The fastest of course is fiberoptic. Remember that the router is the key to a network. if you have a $10 hub, dont expect $100 performance. get what you pay for..
so mainly to speed up performance, on win2k anyway, make sure that all computers are defraged and maintained on at least a weekly basis, and the server on a daily. Do it in the offpeak hours of the work day..