1:

SD <=4GB - byte addressed cards. 4GB is the max (was supposed to be 2GB, but some manufacturers have produced 4GB cards - device compatability is questionable, some may work, others may not)

SDHC >= 4GB - block addressed. 32GB is the max. Not supported in the N800 yet, needs a kernel update.

2:

A 4GB SD (and not SDHC) card that doesn't work today most likely won't work in future. The compatability of 4GB byte-addressed cards is not really part of the SD standard, although some cards obviously do work just fine in the N800. If you have a 4GB SD card that doesn't work it's not likely to be a N800 driver issue but most likely a problem with the card itself.