I have a consumer RAID 5 NAS (network-attached storage) box made by Buffalo called the Terastation. It came with incremental backup software called Memeo. I don't think it does full backup of the entire drive, including OS and programs. It's specifically for user data like things that would typically go in your My Documents folder (documents, MP3s, photos, videos, etc.). It does incremental backup practically in real time. It takes a snapshot (full backup) of the folders and files you specify, and then it watches them. Every time you make changes to files or add fails to a folder, it updates the backup. You can tell it how many versions of a file to keep. So, for example, say you've set Memeo to keep up to 20 versions of files in your My Documents folder. If you're working on a Word Document and clicking the save button every five minutes, and it takes you an hour to finish the document, Memeo will let you roll back to any one of the 12 versions of your document you made during throughout the hour. So it's incremental backup, but it's ALWAYS watching. With that said, its footprint seems entirely negligible. When I was running it, I could not detect the slightest hint of slowdown. It doesn't seem to use any significant amount of system resources. However, I didn't get the Terastation for backup. I got it to be a media server, so I haven't really used Memeo enough to say how well it works in the long run. I checked it out to satisfy curiosity, but I haven't really used it much.
If you aren't averse to spending a little money, in fact, you might just buy a Buffalo Terastation. When I got it, they came in 1 terabyte and 2 terabyte flavors. Not sure what they're selling these days. There's no cheaper way to get into RAID 5 unless you are an IT professional and want to put together a linux affair and happen to have a bunch of unused hardware laying around. The Terastation is easy enough for anybody to use. It gives you hardware RAID 5, which means that your data is distributed across multiple disks, which increases performance, but if any one of the disks fails, you just replace that one disk and the RAID array will be rebuilt with all of your data in tact. You would have to have two drives die simultaneously to actually lose data. And, as I already mentioned, it comes with the Memeo backup software which takes care of all the incremental backups for you.
By the way, to hijack the thread a little, these Terastations are fantastic media servers. They are basically tiny little Linux boxes. It's got native SMB or Active Directory capabilities. Ever since I got the thing I haven't had to leave my PC running all the time. My xbox just pulls media straight from the Terastation. But plug the thing into a UPS. They don't handle brownouts well. If the thing doesn't shut down properly it re-checks the integrity of the array the next time it's turned on which takes about six hours. Streaming media from it during the check doesn't work well -- it's kinda stuttery. But a UPS solves the problem -- especially if you get an APC Smart UPS because then you can run a serial cable between the two devices and the UPS will shut the Terastation down gracefully in case the power goes out.