class 2 is a slow poke.
Class 4 is average, class 6 is alright, class 8 is better but higher price, class 10 is expensive.

16G is cheap but 32G class 2 is 3x the price of 16G class 2 Card. and about 6x the price of AData class 6 card.

But all MicroSDHC will work with the N900. If there is a 64GB then as long it's holding the same format SDHC then it will work. But I don't think they can fit 64GB into SDHC anymore than 32GB.