I'll reply my own post, at least a little: it seems that the maps files, 89 MB of them, take up 431 MB space on disk... interesting. After deleting some 500 MB of mp3's from the a.m. 1 GB card I can download map data again. The problem seems to be, if I want to get larger areas cached, I must a) take only a few zoom levels b) delete some areas every now and then c) get a card that can hold all the maps I need, bloated to 5 times their actual size by stupid Windows FAT arrangement.

Well, should I reformat the card in N800 and use 512 bits allocation size or what? My WinXP only offers default allocation size for FAT/FAT32.