What happens if you have different sources that have different versions of the same package. Say you are getting from both stable and testing, which the one that is installed? Or even different version numbers of the same package, like openoffice 1.1 from stable and openoffice 2.x from testing?

I believe that apt will typically install the latest version, which might really screw up everything else on your install. If you want to run mixed sources, look into apt-pinning which will give priority to one repository over another (i.e. everything will always install from stable unless specifically told to install from testing). Note I've never actually had any luck with apt-pinning as the packages I wanted always tried to upgrade a bunch of things I didn't want messed up, like the kernel...

For stable packages, you can look into backports.org, where some newer packages have been compiled on Sarge and can be installed without hosing up too much.

Aptitude or apt-get will probably keep things straight, but it's not a good idea. You are likely to upgrade packages you didn't want upgraded, and there are other good reasons not to keep both stable and testing or unstable repositories on your sources list concurrently. If you can think of a single reason why you need both at the same time, you need to study "pinning." Otherwise comment out one or the other until you have need of a package from the unused one. Then change the comments around, and when you're done, change them again to go back to your normal source. Don't forget to do "aptitude (apt-get) update" whenever you change them.