Here a few thing you should know:
Dreamcast systems don't need mod chips for back ups. The only somewaht useful modchip for a Dreamcast is to play imports but even then you can always download the Utopia boot disc.
We don't post here where to get games. I actually don't know many places that still have DC games either.
If you want to watch videos on your Dreamcast check out the following:
http://homebrew.dcemulation.com/dcdivxdummies.shtml
http://homebrew.dcemulation.com/gyplay.shtml
http://homebrew.dcemulation.com/dcmovieplayer.shtml
It may look complicated at first but it's very easy.