A backup question

Although I am familiar with backing up my files, but what about my software? I am on a Mac that is over 6 years old now, had several operators and many software upgrades. If it were to crash tomorrow, how would I go about re-installing the latest software. I guess I am a bit confused because we always have to enter previous version serial numbers to upgrade. I am not sure I could track all those serial numbers down back to the beginning. Where would I begin to re-install say CS4? Do I begin with  my current version and serial number? I thought that is what someone told me some time back. I do have all the current CDs necessary to reinstall with the exception of the last upgrade of Quark 8. That was a download, which I burned to a disk after upgrading (I hope that works as a backup as well).

Sounds like a silly question, but I have never been in this situation or gave it much thought.

Thanks