In theory, you only need to install Debian once.
I suggest that you use the Woody installer (provided that it isn't too difficult for you -- Sarge installer is easier) and install just the base system. Then edit the /etc/apt/sources.list so that it points to Testing (instead of Stable) and then do "apt-get update" and "apt-get -f dist-upgrade". The less packages you have installed, the simpler and easier the upgrade will be.
Supporting the distro you use is a good investment if you want to use that distro also in the future. However, because Debian is FREE, I'm not at all sure if anything of the money I pay for the Debian installation CDs will really go to funding the actual Debian project.
If someone can affirm that the Debian project actually gets money when I buy the released CD package, then I'll be glad to buy the Sarge installation CDs once they are published. Debian is the kind of free distro that I'm willing to pay for.
Besides, it'll be endlessly cool to have the full Debian installation CD set, which you can show to all your geek friends to make them green with envy.