Backup server components

I'm going to assume from your mention of 3TB storage that you're looking at backup-to-disk. This is essentially just "dumb storage", so the box really needs to be powerful enough to run your OS and backup tool(s) of choice, and nothing more. Bearing that in mind, the factors you need to consider are getting the data from source to destination (and back again) as quickly as possible, and ensuring that your backup is still going to be there for you when the time comes that you need it.

So we're looking at minimum gig ethernet and - yes - RAID. Storage is so cheap these days that I seriously believe there is NO excuse for skimping on RAID, and in fact I would say that RAID 10 is now the RAID level of choice for most scenarios. Edit: per Oskar's comment, you don't need RAID 10 for this however, go RAID 5 or 6 and it will do the job (see Oskar's own post for further info here). (End edit). You don't want to be in a situation where one of those disks fails and you lose some data that you need to get back, do you?

If budget allows I would also include a tape unit. I like tape; it's robust, cheap, high capacity, portable, and will provide you with a second line of defence in case the server lets you down. Make it an external one so that you can connect it to another box if the worst does happen.

Working from there you're looking at a scenario where you do a full backup each weekend (making certain to backup everything, not just what you think you need), daily differentials, and also run the last full off to tape as your second line. So figure how much data you have, how much of it changes, how many weeks you want to keep, and calculate your storage requirements from that. Double the result because storage used is going to grow, and I think you'll be doing good.

3TB of space, I'd say get 4 or 5 1TB drives and build a RAID5 with optional (highly recommended) hot spare (or go RAID6). As 1TB drives are as close to twice the price of 500GB drives you can get - there's little use going for 500GB drives in my opinion.

Get a separate (or if possible two) smaller drives for running the OS, would be best to run those in a mirror/RAID1.

You do not want to loose data and every disk WILL break sooner or later. The question is when...

...doing RAID5 is normally easy within the OS itself - no need for a card.

Lastly, if you're going to protect company data no matter how small the company is - you do not want to build the machine yourself. Buy a preconfigured server by a big-name manufacturer instead like Dell or HP.

I'd go with a pre-integrated server for backups. Something with support. Unless I was only backing up my personal stuff at home. You don't want to have anybody looking at you a little sideways when they get to figuring out there's something gone wrong with the backups and how it might have been your integration.

If you're going to build your own: Mirror the OS/Applicaion drives. RAID 5 the volumes you're using for backups, preferably with a nice hot spare.

Don't totally dismiss tape, as an option for your long term data storage needs. If you're doing quarterly, year end, or month end backups. Also it's way easier to get tape off site than disks. Backups aren't good backups if they can be destroyed in the same incident that destroys the originals. So contemplate the placement of your backup server closely. Will a single lightening strike, spilled coffee, contractor with a carpet steamer (yeah it happened to me) destroy the server, and the backup server? If yes, then you should think about if that is good enough backup.

one thing that i think is essential for a backup machine like this (especially if you're not going to have a tape drive, which would be a really bad decision IMO) is to have hot-swap bays for the drives.

this would allow you to quickly swap drives if any die (essential if you use RAID).

it would also allow you to upgrade the drives easily as larger drives become affordable - e.g. a quick and easy upgrade to 2TB drives.

and finally it would allow you to use disk drives in a similar fashion to tapes - as "data cartridges" that can be installed in the machine, written to, and removed to place in a safe or off-site. you'd be trading off the extra fragility of hard disks versus the cost of a tape-backup drive (external LTO-4 drives start from around $1500 AUD - which would buy a LOT of 1TB drives)

oh, and look for a motherboard with 8 sata ports rather than 6. 2 for the operating system drives (RAID-1 mirror), and 6 for the data drives whether they're in RAID or not. 8 is quite common these days (at least on AMD chipset boards which i'm more familiar with than intel boards)