Upload & Sell: On
Maybe try setting up an online account with Adobe using your CS3 software as the basis. See if that works. Others will know better, but I think once you are in there, Adobe keeps track of the software you legally own so future upgrades are easier. And you don't have to worry about losing the CDs.
As an aside, I just bought a new laptop and installed CS5 that I bought as an upgrade to CS4. My original software was CS2, bought way back when you could get Photoshop for something like $250 using the Adobe Elements serial number that came with the 10D. The CS5 install recognized that it was an upgrade, and it asked for the previous serial number. I wondered how far back they would make me go (all the way to the original CS2?), but the CS4 serial number was enough.
So my old machine is running both CS4 and CS5 (since I never tried to remove CS4), but my new machine is running CS5 as a clean install.