You could have read the thread on this forum discussing the latest firmware and learned that SDHC is supported natively by the latest 4.2007.26-8 firmware!

However, the official Nokia SDHC support omitted the high-speed support that is/was previously available in the community patched kernels, hence a version of the 4.2007.26-8 kernel is now available with the relevant patches.

Unfortunately, the N800 hardware has a tendency to corrupt SD/SDHC cards, and even render cards inaccesible. This may be independent of kernel version, as the cause is so far undetermined.

I personally use the community patched version of the 4.2007.26-8 kernel and haven't had any problems - I did have unexeplained filesystem corruption with the official 4.2007.26-8 kernel.