Can't be done on the N800 as cfdisk was only ever ported to OS 2005. You'll need a Linux PC and cfdisk or sfdisk to partition the SD/MMC cards. As long as you have access to a PC, even a Windows PC, your best bet would be to download a LiveCD ISO (eg. DSL - Damned Small Linux) and boot from it. You may also find the LiveCD useful when it comes to flashing your N800 with updated firmware as the Linux flasher has far more functionality than the Windows flasher.