Any tablet can do most of the old systems, but add in Dreamcast and things get weird... Some games will run on a potato and some need more "power" I think it's down to the emulator and it's still being worked on.
My best advice is get a larger screen like 10 inches, have blue tooth for controllers, 2GB's or more of RAM. Internal storage is of course more is better. Make sure it has a MicroSD slot of course. Also get the latest version of Android you can get your hands on, most tablets do not get any sort of updates unless you spend big bucks on like a Galaxy Tab what ever is the newest one.
Also get ready for a pain in the butt, unless RetroArch has gotten easier to work with on Android tablets, my tablet has a weird bug... When you first install it the text is set to like 14 point, but my tablet is a 2K res so literally the text just looks like a few weird lines on the left side of the screen lol