+ if you have mugen cover + you will tear up any size (even 32 MB or lower) microSD card + similarly cheap SD (fullsize) reader, you may construct trivial microSd->SD adapter, that will fit entirely inside N900's body.
Not much reason to do so, though - SDXC cards are still ridiculously priced, and will shrink to micro sizes very soon.
Out of curiosity - no software problems in handling SDXC on N800?
Well, the proprietary exFAT filesystem isn't supported for obvious reasons, but reportedly they work fine after reformatting to something else. There are other threads here with more details but in summary the cards are electrically identical to SDHC, and the only software-side difference is that the size in blocks is a 22-bit value for SDHC which even the Diablo kernel supports just fine.
They're still a bit expensive, but I might hold out for one of these when the price comes down if someone can confirm they work.
As far as I understand all SDXC cards will work, when they are released. This should see us up to 2TB. The only issue should be they will not work out the box as will require reformatting.
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.)