http://store.apple.co...elect?mco=MjE2MjYyNzA
They are available in 16GB, 32GB and 64GB and all sizes are available as wi-fi only (you need to be near a fixed wireless point with permission to use it to be able to connect to the internet) or wi-fi and 3G (you can connect anywhere with a mobile phone signal)
You've not said if yours is a wi-fi only or wi-fi and 3G, if it's wi-fi only then a 32GB wi-fi and 3G is going to be a "better" model and so would any 64GB model, if yours is wi-fi and 3G then the 64GB wi-fi and 3G model would be better.