Basically if you are running a small company (less than 10 pc’s on a network) with only say 3 programs that are been used, then there is absolutely no justification to have a server installed. A server will serve the function better as you have said but a decent desktop with a 64bit operating system will do perfectly fine. To justify my statement you merely need to take a look at LAN gaming.
As you will proberly try bring out at this point, file sharing is diffrent, the fact remains that we have Phenom x 6′s, I7′s and i5′s in the market at this stage. Combine the cpu with a 64 bit and you have access to more than adequate ram.

So in short my guideline is , anything under 10 pc’s on a network with basic (this is a key word here) programs. IE : one POS system , excel and pastel and there is no need for a server, just get a decent desktop with 64 bit operating system.