playing nes games on a dream cast

You can find some info here: http://www.dcemu.co.uk/
Go to the "emulators for DC" section and you will find emulators for many systems. In order to make these disks work on a dreamcast you'll need a tool to make them bootable (some emulators come with such tools or have a guide on how to make bootable disks on their website).
Be aware that Dreamcast is not fast enough to emulate newer systems at full (or even playable) speeds. This means that you can't run any N64 games on a Dreamcast and games for SNES may not run at full speed.