You should be able to use NTBACKUP to schedule / perform your backups. I haven't used it to burn to DVDs but I use it on all of my machines to back up across the network a server. I'd start by making a complete backup and then set the software to make incremental backups of just the directories you need. I'll be interested to hear how it goes! Good luck.
If your DVD writing software supported it you could use the NTBACKUP as suggested and then have a batch file copy things over for you.
Some software like Symantec's Ghost 9 will write backups directly to your DVD for you. However it doesn't seem like this is a job that can be directly scheduled with the software. (I'm sure a batch file of some sort could work around this limitation.)
For a totally automated solution you would need a disk changer to swap out backups. Maybe if you could load DVD-RW discs on a carousel of some sort you could have a completely automatic system.