Content server backups
RAID is not backup.
Now that that's out of the way, if you have 15 servers which only hold content, and each one is 250 GB, it's time to ask yourself some questions.
0) Should the data be centralized?
Unless you just happen to like managing the storage on 15 machines, you should probably shoot for pooled, managed storage. This does come with a cost, though, Storage is cheap. Managed storage is expensive. If you don't want to (or can't) manage it centrally, then you need a tape solution. The cheapest solution would be one server with a large amount of disks (in a RAID configuration) attached to a pretty large tape changer (ideally, since you don't want to manually change tapes every day, I assume). You could also get 15 tape drives and attach each one to a server, but that's dumb.
1) What is your data retention policy?
In other words, are you going to keep the data forever, or for a limited period of time
2) What is your size delta?
How much does your data change per day? That needs to be factored into your future storage plans. Equipment purchases are not just IT related. Accounting needs to be factored in. If you depreciate your purchases over 3 years, you need to purchase storage that will last you 3 years. Do the math or pay the price later.
3) Where are you going to put it?
15*250=a lot of data, as you mentioned. You've got to figure out where you're going to put it. If you want it to be "live", you've got to get a storage array of some sort. If you want to back it up to tape, you're going to need a tape changer attached to a server with some big storage.
4) How much of the data is a copy of the other servers?
If you centralized the storage, you have the opportunity to invest in a storage array that has "data deduplication", which saves tons and tons (and tons) of space. Essentially, if a file over here has the same data as a file over there, the data is only stored once, and a token is stored instead in each place which is smaller than the original data. Solutions that provide this are expensive, though.
Please tell us more about the current network topology, data characteristics, server specifics, and whatever else you can.
RAID isn't a backup. Say it with me, and repeat it to yourself again and again. RAID protects you from equipment failure, but not disaster.
Whatever you do, having a backup kept offline is essential. If someone can maliciously or accidentally trash all your backups, because they're all online and accessible via the network, your backups weren't really backups. (Read up on what happend to "avsim.com" when they got hacked if you want to see what I'm talking about.)
Raid will only provide you with backups in case of hardware failure. What you need is backup software to make a duplicate copy of all content on another server, preferably in a different geographic location.
I'd buy a backup server with a few 1TB drives and backup everything to the backup server.
Took this answer from a previous question about backups as I believe it still applies here (FYI it was my answer, not someone elses):
Depending on how much you need to back up I would recommend the following:
1.JungleDisk / Amazon S3 - Works VERY well.
2.RSYNC to a remote machine also works very well. CRON job every XX hours.
We back up almost a TB of data to Amazon's S3 cloud and have a "warm standby" at our colo backing up from the master several times a day (via rsync). The cost for transfer/storage on Amazon S3 is extremely cheap. (ie. cheaper than burning to a DVD but not cheaper than backing up to HDD. I know some folks who simply plug in a 1TB UDB "My Book" or something into the server and back it up weekly/monthly. Depending on your needs one or two of those might be the cheapest solution for you.
Now that's just talking about DATA backups... not backing up the server itself...
Depending on your needs, Norton Ghost or even Acronis (http://www.acronis.com) might be of help to you. Things like Norton Ghost tend to rely on your ability to be able to actually turn OFF the computer to make the backup. Some of us don't have that luxury but if YOU do then Norton Ghost is a VERY good product.
RAID should not be used as a backup solution. I'd get external drives or setup a backup server with something like BackupPC and then rotate the disks and store at least one copy off-site.