When building out your data center with servers, you really have 3 options. Either buy Tower based servers, rack servers or blade servers.
Deciding on which type of server farm you want all depends on the real estate you have to house them, your long term growth plan and of course your budget.
Tower servers are generally on the lower end price-wise but can grow quickly into a very expensive solution.
As I had mentioned above about real estate, tower servers take up a lot of space because they are individual systems that also require individual monitors, keyboards which would also require a place for these peripherals to set on.
Rack servers are commonly used in medium to larger sized data centers. These servers are normally mounted inside of a rack or medal cage that, for example, would be 42″ tall and 19″ wide.
Rack servers are very expandable; some include 12 or more disks right in the chassis and support for four or more processors, each with multiple cores. In addition, many rack servers support large amounts of RAM, so these devices can be computing powerhouses.
Blade servers are great when it comes to packing as many servers as possible in a small amount of space. But the down side is that each blade server is extremely limited in it’s ability to expand. For example if you are wanting more PCI/PCI Express slots. You will have to purchase a chassis expansion module.
Blade servers are like big PCI cards. You rack mount your chassis and then slide your blade servers into the available slots. For example a Sun chassis 6000 has 10 slots for blade servers. So adding a new server to your environment is really no hassle.
Take a look at “The pros and cons of tower, rack, and blade servers” by Scott Lowe for his view on which server is best suited for your server farm.