no chance to format anything to ext under windows.
you need a linux live cd.

in my opinion gparted is genius and i always use it to resize some partitions and format them. it has a minimalistic gui. but i don't know if it detects card readers...

mhm... otherwise you could download any other live cd... for example ubuntu 9.10... it should detect all of your hardware correctly and there shouldn't be any problems... and if there are any problem... just google... the ubuntu community is very big and you'll find several solutions for most of the problems just by searching for "ubuntu explain your problem in short" ^^

but it's strange that the n900 doesn't recognize the FAT32 card. maybe you want to try a reformat to FAT32 first.