Just to add my experience: I have 8GB Sandisk MicroSDHC (with Topram microSD to MiniSD adapter) in my N810 and it works great.

Future consideration: If you plan to upgrade later to the "N900", you have better luck (*cough*) to use microSD card with it than MiniSD. (But of course it is still long time until we are there, and flash memory capacities go up and prices go down... so you may not need to worry too much future upgradeability?)