I'm not sure if you've already read this but have a look here:
http://www.dcemu.co.uk/vbulletin/showthread.php?t=11086
For a NES CD image packed with roms (homebrew only) and loads of extras, check here:
http://www.dcevolution.net/goto.php?id=65
For a SMS CD image packed with roms (homebrew only) and loads of extras, check here:
http://www.dcevolution.net/goto.php?id=54
With both the above images you can make a 'romdisk' with your own roms on there - just boot the CD in your DC and swap in your romdisk (check each image site page links for info for making a romdisk).
SNES and Mega Drive (Genesis) CD images packed with roms and extras are coming soon.
There were only 3 official Playstation Emu releases known as Bleemcast!, there is an illegal beta version of Bleemcast! floating around the web but it does the Emu no justice at all, with the official releases the games look better than on an original Playstation (well worth purchasing all 3).
There is a N64 Emu for the DC but it's more of a 'proof of concept' and does not run game roms (yet?).
Hope that helps you get started and welcome to the scene!.
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