Single processor Quad core
This kind of servers will give slight enhancement than a desktop PC. Will feature ECC RAM and better cabinet and its more reliable than a desktop. Normally Used for connecting 5 -10 Client PCs.
Single processor Xeon
This will have server processor like Xeon and will have dual network card, basic server management, more memory slots and possibility of both SATA and SAS interfaces. You can have RAID features. Ideally suited to 5-25 clients with simple application.
Dual processor Xeon
This platform gives all the features of a professional server. Dual processor, SAS HDDs ,more memory, server management features , suited to 25-100 clients or users.
These are Departmental servers and can be used as high end server used for Database applications,ERP etc. This can populate up to 4 processors and each processor can be of six cores.