Which models do you have in mind, the 2xx5 series ? Unfortunately Garmin doesn't publish any specs, but based on the pricing of the 3xxx and 2xxx series, the 3xxx must use more expensive components, the higher-resolution "iPhone like" screen for a start.
Pulled these internal storage numbers from a number of sources, so don't ask were I found them as I simply don't remember and I can't guaranty them 100% :
Nuvi 30/40/50 = 2GB 24x5 and 25x5 series including the 2595 = 8GB 3400 = 8GB
For some of these model's I've seen numbers in the 7+gb range but I'm betting that's 8GB less the system overhead.
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I think it's also region dependant. I know for a fact that the EU 35xx series has 8GB internal memory, whereas the AU 35xx series only has 2GB internal memory. It's most likely similar for all the nuvi series as the maps for AU & NZ are smaller in file size than the EU and US maps.
It does kinda suck tho that Aussies pay more for a 3590 that only has 2GB capacity, than the EU 3590 8GB version.