Overloaded Server

I wonder if there is a rule of thumb on how many applications should be on one server. I have a Dell poweredge 2400, with a RAID controller, 750 mhz PIII, 512RAM, with win 2k server and AD. The single server plays the role of an Exchange 2000 server, ISA server, SQL server, File server, and print server. I think its overloaded ’cause its really SLOW. Does anybody have an opinion?