Retro I think greatly depends on how old you are, for me retro is Arcade, C64, Spectrum, ST, Amiga, and the 16-bit consoles. Games that we can't really get these days.
32 Bit though, I mean what consoles were 32 bit, and what games are actually worthwhile to remake

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I think there should be a destinct guideline, maybe just any game made before 1998, then it could be a standard 10 year old thing, if a game is more than 10 years old then it's retro perhaps. Personally I like to concentrate on older games, games that I can run on an emulator on my DS, Speccy games are ideal for this.