You don't need a super-fast CPU for a file server. However, it can be a good idea to have more than one processor. A single CPU can be kept busy doing parity calculations (needed for RAID 5), but if you decide to use RAID 6, there will be more parity calculations and you will need more CPU cycles.
My first file server sports dual Pentium III 933 MHz processors. I can see 100% CPU usage during RAID array rebuilding, so I recommend something faster. My second file server uses dual Xeon 2.8 GHz CPUs with Hyper-Threading, and I have never seen both cores at 100% CPU usage.
A 2 GHz dual-core AMD processor will probably be sufficient. Of course, newer processors tend to be more efficient, so if you are going with a more modern platform, you'll likely save energy as you enable better performance.
If I were buying a new processor for a file server today, I would pick a slow, cheap AMD Phenom II. The reason is they can be had inexpensively, the motherboards for them are priced reasonably, they run cool, and the chipsets for the motherboards generally support ECC and Chipkill.
No matter what hardware you buy, you should have a UPS to deal with power failures. You can get by with a cheap one, but a higher-quality unit will likely pay off in the long term. At a minimum, you will want to be able to shut down your file server in an orderly fashion should the power go out, which means a minimum of three to five minutes of power. Another nice feature of most UPSes is that they have a built-in surge protector.
Obviously, there can be a wide range of costs in building a file server depending on how much storage you want and what hardware you already own. Here are some typical costs and options:
- Case: $150 for a case similar to my Cooler Master Stacker 810. Look for the ability to hold lots of disks
- Power Supply: $50 for a 350 W or so 80 PLUS power supply
- Disks: six 1 TB disks, roughly $80 each
- Operating Drive: Free, assuming you can find a 10 GB drive lying around
- DVD Drive: $20
- Motherboard: $100 is plenty for a dual-Opteron motherboard with 2-4 GB of ECC memory, assuming you don’t have some spare hardware. You could start with a dual Pentium III motherboard, which is cheaper than dirt. I happen to have several gathering dust. Expect to pay $150 for a new motherboard with a warranty
- Memory: $50
- CPU: $100
- SATA Controller Card: $100
The total cost is roughly $420-$620, plus $540 for the hard drives. This will make a 5 TB RAID 5 file server that can easily be expanded to eight or more drives. If you are building your own, I suspect you will have a fair number of the parts already lying around. This is cheaper than most NAS boxes that hold four or five disks and will be faster and far more flexible.