The N900 supports the micro-SDHC standard, which means that it's supposed to flawlessly work with 32GB microSD cards.

However, those cards have only been announced by SanDisk, but they are not for sale yet. As far as I remember, SanDisk announced 32GB microSDHC cards in February, but those cards are not available at this time.

For now, 16GB is the most you'll get.