First, a bit of information on a NAS. The filesystem of a NAS is not exposed to the network. This means it is different than accessing files on your local computer. On your system, if you request a file, it looks it up in the inode table and pulls the information from the sectors themselves. When you request a file from a NAS, it simply says "send me this file h:\path\file.iso". Because of this, multiple computers can connect to this share and it shouldn't matter what OS you use to serve/connect with. So, you could be running a Linux/Windows server with Mac, Windows and Linux clients. That being said, the file system has nothing to do with the clients.
I'm running my linux server (CentOS 5.6) with two EXT4 formatted RAID arrays. I share via a service called "Samba". My clients are mostly linux (Fedora 14, at the moment), but do include Windows (XP, Vista and 7) and a lone MacBook Pro. They have absolutely no issues with reading files and I've been doing this for over 2 years.
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Operating system:
What operating system do you intend to use for the server? If this is something that you want for "set and forget" storage, I would suggest Windows, preferably the "Home Server" edition. If you are looking to learn a bit and don't mind tinkering, I would highly suggest a Linux flavor. There is also the option of a "NAS operating system", such as FreeNAS. As the name suggests, it is free.
Case:
This would be a better choice and you can fit a crazy amount of drives in it if you are
crazy like me. The picture linked was my server a long time ago. The case is cheap, well designed and well ventilated.
Hard drives:
For storage,
I would suggest these. You can get 8 of the Hitachi drives at the same price as 4 of those Seagate drives. At very least, it saves you a large chunk of money. If you are getting a real hardware RAID controller, do
not get any Western Digital Green/Blue/Black drives or any Seagate Green drives. These will cause you massive issues, as the disks will constantly drop out of the array. Any Hitachi or Samsung drive should work fine.
Motherboard/processor:
If you are going to use this purely for storage, what you have selected is fine. If you are going to use it for rendering, a testing environment or anything that uses the processor heavily, I would suggest using a full blown processor and board.
RAID level:
This depends on how many drives you will have and what you are using it for. If this is simply storage, RAID 5/6 will work fine. If you are going to be hammering the drive hard (virtual machines, databases, etc), then RAID 10 will work better. For light use storage, I would suggest RAID 5 for 3-5 drives and RAID 6 for anything more than that. RAID 5 will allow for one drive to fail and RAID 6 for two to fail without catastrophic data loss.
RAID card:
This one is a bit interesting and depends quite a lot on what you need now and what you may do in the future. If you aren't going to ever have more than 8 drives, I would suggest a Dell Perc 5/i. These can be had under $100 and is full blown hardware RAID.
I wrote an article on the front page for mine and there are
very good threads on the internet. If you are going to have more than 8 drives, this gets tricky and a
lot more expensive.
See my post here about how I solved this. Scroll down from that post for cool pictures.
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So, to help, answer these:
What do you intend to use the server for now, and in the future?
--Is the sever purely for storage, and nothing else? Streaming content (watching movies, listening to music, backing up) counts as "storage".
How many hard drives do you plan on having now, and in the future?
What is your budget for a RAID card?