Windows 7: Dvd-r

Blank DVDs aren't really a good system backup method. You would be much better using an external hard drive to maintain a system backup.

There are also DL DVDs, which hold 8.5GB each. The main disadvantage to them is that usually have lower write speed than the single layer (4.7GB) types.

There are drive backup utilities (such as Acronis True Image) that can make a drive image that spans any number of DVDs. I've never gotten that to work by writing directly to the DVD burner, though. One or more of the DVDs always seeems to be corrupted. I've had better luck writing to another hard drive, splittiing the archive into DVD-sized chunks. I burn the chunks to DVD later.

Even using compression, backing up 40GB to DVDs is something I wouldn't care to do.

I agree to that. I have several DVDs of personal data, but I have my backup images on an external hard drive. The HDD is more depenable and works a lot faster than reading a DVD.