Managed to read through it and now I think I understand it a little bit... about changing the sources from lets say a .de to a .us - all this affects is the speed of the download doesn't it?

I have read the APT howto (whether it registered is a topic for debate) but I am just a bit too wary right now. Anyway, assuming I don't change the default sources, is it okay if I do an apt-get update and an apt-get upgrade?