Bodger is right. The CDR is the cheapest way to go. Your software (Adaptec) is the most common around, and works well.
When you become familiar with the system, you will realise that each time you back-up material to the CD, you have to leave the CD session "open" (i.e do not close it) otherwise you cannot write any further info to it and the weeks backup is all you will have, definitely a waste of the CD. Your software instructions will clarify this procedure. Basically, this means you can add more material as many times as you need until the disk is full (the 650Mb limit).
However, this may also mean that the CD may only be read on the equipment writing to the CD (your writer), until the disk is closed (when full). A closed disk should be able to be read on any CD reader. That is the only limitation I am aware of right now.
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When all else fails, read the manual!