How do I chop up ISO files?

I want to back up a couple of iso files, but they're too big for one dvd+r. How do I chop them in half?
 I figure I can just put them into .rar files and extract them into the same directory for when I want to watch them again ( I use daemontools for that)

Basically, I need to do some data cleaning (I'm using almost all of my 143 gb usable HD space (the recovery partition is looking pretty useless right about now)), and 18 gb of space isn't small no matter how you look at it.

>>2
The Recovery Partition is only 6 gigs (with 999 megs free space), so that doens't really solve my problem.

>>3
But I'm a cheap-ass. Seriously though, I would rather back it up on a dvd+r dl (how do I show that a dvd is dual layer in the short form? is is dvddl+r?), and I'd rather get more space by using good date-management as opposed to getting an external hard-drive.