I personally recommend the "oid" emulators, on a decent or over clocked kernel they run VERY smooth, as far as roms go I'm unsure if purchasing them makes them okay to possess but I believe discussing where and how to download them is considered piracy and therefore not to be discussed here. As far as I know in order to legally possess a game backup (which is all a rom is) you must own the original copy for the console/system. If you have trouble finding the gbcoid apk pm me and I would be more than happy to email the apk to you.
P. s. I am not trying to be mr. Goody goody lol I just know that it is rare for someone to read site rules and terms when signing up with the site so I don't want to see someone who is unaware of the consequence get banned.