Creating backups on DVD-R or DVD-RW allows you to store up to 4.8 gigabytes of data (or 2.4 if you use single layder DVDs).
The main advantages:
- low cost
- archival, by default
- widely supported, and readers are common
The main disadvantages:
- slow write speeds
- limited capacity
- data is easily damaged
If you are going to backup to DVD, get an SATA DVD burner.
Chances are, you're only going to do a data backup, so, make duplicates of all your installer CDs and DVDs first. Make a disc with all the downloaded installers, and all the serial codes.
Then, backup the data. You may need to partition your data on the disk, and set up different backup jobs, to spread the backup across multiple DVDs. Prepare to spend a lot of time waiting.
Another disadvantage is that you can't always choose backup software, because the burner may not work with generic DVD burning software.
That all said, a DVD is very light, and easy to mail. It's a great way to make a weekly backup of a large project that can be sent off-site "just in case". It also gives your client something solid in exchange for paying their weekly invoice.