As i remember N900 can only use SDHC cards so for 99% - No, those cards will not run.
EDIT: It looks like SOME of the slower (?) SDXC cards can probably work on N900. Quote from Wikipedia: "Existing SDHC hosts will only support the SDXC cards at up to UHS104 speeds" But the other problem will be filesystem on those cards...
As i remember N900 can only use SDHC cards so for 99% - No, those cards will not run.
EDIT: It looks like SOME of the slower (?) SDXC cards can probably work on N900. Quote from Wikipedia: "Existing SDHC hosts will only support the SDXC cards at up to UHS104 speeds" But the other problem will be filesystem on those cards...