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.