On which system? On most systems default FAT32 cluster size is 4KB no matter the size.

Are you sure about this? is this result of some practical test?

While I'm not sure anything below 4KB block is supported with FAT32 at all, pure math says limits should be different. 4GB fits inside 32bits even when you address it by individual bytes, here you are addressing by clusters so why you can't have 512 bytes sized cluster with 4gb card? That should give you 2^32(=4294967296)/2^9(=512)= 8388608 clusters which should definitely fit into 32 bits of FAT32 limit. So in theory you could have 2^(32+9) bytes big card for 512 cluster size. Or if inforamation here http://www.cknow.com/ckinfo/f/FAT-Fi...tionTable.html or here http://en.wikipedia.org/wiki/File_Allocation_Table is true (and it probably is) and only 28 bits are used is still should allow 2^(28+9=37)=128GB card to have such small cluster.

Anyway making the cluster size small will make FAT table quite big. Generally it may not be very good idea to choose size below 4KB on gigabyte sized cards. Check this
http://www.storagereview.com/guide20...partFAT32.html
4GB card with 1KB block would give you FAT table size of 16MB and 512 bytes cluster will take 32MB.
Since most devices are not caching writes and keep FAT table in RAM and update it on every write this could make such card very slow and even not working in devices like PalmOS handhelds or digital cameras (due to device RAM limit).

But maybe linux is clever enough and handle big FAT tables sensibly so in N800 this could work?