From experience it is difficult to say what it happen; because of the different variables. We started with started with a HP 385 with 19G Ram. We were able to put 25 users on the box before they start screaming. I highlighted screaming because that is where the problem is. Dispite the spec. of your server or whatever resellers tell you once your users start complaint about performance you must stop.

Base on your current spec. and you wanting to have 40 clients. I would suggest you add more ethernet card it will help, but a lot depends on the type of user and the applications they are running.

Also once start avoid doing anything on the sever during working hours e.g building clients - this make things worst.

It might seem like a waste of resource but, you want to avoid possible bottleneck. Think about it for a minute - asuming 40 desktop PCs, running xp, office 2003, plus core business apps. and all the other bits - using all these at peak time; 1 nic will become a problem.