So now that your box is assembled, I recommend Knoppix Linux, which is a bootable live system on a CD or DVD, to test it. This will check that Linux recognizes all the hardware. With Windows, almost all drivers are written by the manufacturer and are tested with Windows. However, with Linux, most vendors don’t supply drivers and rely on Linux volunteers to write the software based documentation.
More enlightened manufacturers supply Linux drivers that they support. For example, all Intel 802.11x wireless chips include Intel-supplied drivers. I recommend supporting manufacturers that support Linux on their hardware.
Several-year-old hardware is almost certainly well-supported by the Linux community. If there were any bugs with the drivers, then they stand the best chance of having been fixed.
It is possible that the latest Linux distributions will support your hardware, while the slightly-older Knoppix distribution will not. This should only happen with very new hardware. Just burn a disk, alter the BIOS to boot off the optical drive, and your computer will run Knoppix.
One boot option is to run memtest86+. I like running it for a day or so to be sure the system is stable and there are no memory errors. There is no point in installing software when there are hardware reliability issues.
There are several choices for operating systems that support software RAID, such as Microsoft Windows Server operating systems with support for RAID 5. You can even tweak Windows XP to support RAID 5.
However, I don't recommend Windows for several reasons. First, it is expensive. Windows Server 2008 costs start around $999. Another reason is that Windows does not remain as up to date as other operating systems do with respect to RAID support. Finally, Windows is (in this writer's opinion) likely the least secure and reliable operating system, both of which are very important for file servers.
There are many ways of measuring security and reliability, and you can find many biased reports, some of which are even sponsored by the vendors. I have found a good report on security at The Register. Although it is from 2004, the main points remain true today. They found that, for the top 40 security bugs, the average severity was 54.67 for Microsoft and 17.96 for Red Hat Linux. I recommend anyone choosing Windows for their file server to read the report first.
Next are the various versions of BSD: OpenBSD, FreeBSD, and others. They don't cost anything and are reasonably secure and reliable. The biggest limitation is they aren't as modern as Linux with respect to RAID support.
OpenSolaris doesn't cost anything and is also fairly secure and reliable. It does have limited hardware support. On the other hand, it has ZFS, which is currently the most sophisticated, reliable, and robust file system. Plus, it incorporates RAID 5 and RAID 6 functionality. It isn't as popular as Linux, but if you are familiar with it, it is a very good choice for a file server.
Finally, there is Linux, which also doesn't cost anything and is both secure and reliable. It has great hardware support and supports RAID 5, RAID 6, RAID 10, and virtually all other forms of RAID. Linux is evolving quite rapidly, so new hardware will quickly be supported and new software features will be quickly added. When you update a Linux system, you don't have to reboot it, so Linux systems can run continuously for months or even years at a time.
There are many different Linux distributions. Some, like Red Hat, offer better long-term support than other distributions. Others, like Fedora (which Red Hat distributes), are geared for quickly incorporating new software into the distribution. Ubuntu’s main quality is its user-friendliness, while it is the distribution that is the most popular. You can read about the top 10 distributions here.
I picked Mandriva Linux since there are releases twice a year, the support lasts several years, and it has all the features I care about. However, any recent Linux distribution can be made to work. For documentation information, check here. There is a very good introduction to Mandriva guide, which is recommended reading before installing Linux for the first time.