Older hardware does not support XC. I dont know if the N800 will or not, but it predates the XC specs. According to wikipedia - XC compatible devices did not start appearing until 2010.

At least the N800 does handle SD version 2 - so SDHC up to 32GB... and I think the hardware could in theory support XC - but not at higher speeds and possibly not without software/firmware support. I'd love to be proved wrong

(As a side note _ I have several USB adaptors that support SDHC, but really annoyingly not SDXC.)