With the way that most popular server systems are going of late, with higher and higher hardware requirements, it is getting a lot more difficult for small businesses to find a reasonable deal when they want to set up a server at their office. It is still possible to install the latest email and file server software from Microsoft on an under-powered machine, but if you do a search on the internet for "Server 2008 slow" or "Exchange 2007 slow" then you will be able to read about the many people that are suffering with this new software even when running on reasonable hardware. A few years ago one of my clients decided that they wanted to upgrade to a better server and I advised them that I could install OS X Server onto one of their machines and attach a RAID tower to it. The main reason for using OS X Server was to take advantage of the various open source components that Apple had decided to include for the email and database services. The server has performed well over the years and even when spam filtering was added to the mail service, I did not have any complaints from the users like you would find in the above internet searches.
There has been a rise in the availability of low powered processor boards that can be used in various small cases, and they are just right for a small server that you can hide in a cupboard, but these machines do not have enough power to run the main server systems available from Microsoft. Microsoft do have a server software that is supposedly designed for these systems, but if you do a search for "Windows Home Server slow" then you will see that it seems to still have the same problems.
The best way forward now seems to be to pair one of these systems up with a free unix based operating system like FreeBSD, CentOS or OpenSuSE. With one of these systems installed you will be able to remove all the services that are not needed which will drop the amount of memory used down to a minimum. Another good thing about each of these operating systems is that they are constantly updated and if an upgrade is needed after a few years, then it can be freely installed and there will not be any worries about compatibility because all the software will be open source so there will always be a way to update it even if some custom modifications have to be made.
There is one problem that I am seeing a lot of nowadays, and this is older servers that are running old software with no way to upgrade it without the client having to spend a great deal of money. The use of open source software on servers in the future will help to alleviate this problem and will help to reduce the number of servers that are left to rot with no chance of anymore security updates.