Before you buy server
Branded or Unbranded : You have two choice, If you are looking for a mission critical applications and you really need performance and you are going to run your servers 24x7 you must opt for Branded Servers. You can choose from IBM, HP, Dell Servers.
Unbranded servers are good for small business or companies to run non-mission critical services like and budget is a problem. You can still use non-branded servers for mission critical use or running 24x7. It all depends on how your server is build, what components are used.
Rack or Tower : Rack servers can save lots of space compared to tower. If you are planning to co-locate your server in ISP data center you have no option but to buy Rack Mount servers. Rack mount servers are bit expensive compared to tower servers.
If you need only one or two servers and you do not have dedicated server room and rack. You must buy tower servers.
Hardware or Software RAID: Operating system like Windows 2003 or 2008 provides you option of having software RAID where you can do mirroring or even RAID 5. But this will effect performance. If you are looking for server for small set of users and your budget is limited, you may go for software RAID. Software RAID is created after Operating System is installed.
To setup hardware RAID you need a raid controller. Many server motherboard have onboard RAID controller with RAID 0,1 or 5. Hardware raid donot require processor resources hence you will get better performance. You may get features like Hot Swap which allows you to replace disk while server is running. You must opt for hardware RAID if you are using your server for mission critical applications like Exchange server or ERP server.
Running servers 24x7: If you are hosting mail server; website or running a mission critical business applications like SAP you may want your servers to run 24x7. You will need to plan and design your data center to run servers 24x7. It's not only running servers at 24x7 but your entire data center would run 24x7 to maintain availability. The first thing is providing enough power backup, cooling and redundant links. You cannot ignore security. Your server should be protected behind firewall which is configured well. Only required inbound ports should be opened. You must monitor event logs, disk space, processor and memory usage.