Everyone here is wrong

The best CD's to get are 99 minute (870MB) CD's. They are ultra-high capacity CD's.

All CD's have a shortcoming compared to the Dreamcast's GD-ROM's, and that's the size (capacity). The Dreamcast disks can retain 1GB of data. People making backups have to remove or compress a lot of media in order to get (most of) the games onto the smaller CD format. With ultra-high capacity disks, not as much squeezing needs to be done to get the games onto backup media, so the game is going to be fuller and truer to the original. There are some games that I have only seen released on 99 minute disks (but I'm new to backups). These backups are usually contain "99MIN" in the title.

Here's a place they can be ordered online, but I hear they are not as hard to find in-store in Europe: [links not allowed, apparently. Look on deal extreme.]