ok first of all congrats on your choice of console
Now to the answers to play backup games you have no need to get a mod chip for the dc. However you may need a bootdisc for some games (theres selfbooters and non selfbooters), you can find that at this webpage.
You cannont copy dreamcast games using your pc and nero, you need some extra hardware (coders cable or a bba (i think) and a knowledge of how to downsample all the files and stuff. So untill you learn some more just download the games.
For the emulators and dcdivx, dont get so excited it may be nice but there are only a few which run full speed, and dcdivx you have to most of the times downsample your divx files. However that said there is some good emulators around. For help with this goto either
www.dcemulation.com or
www.dcvision.com
I think thats it
good luck and have fun.