Assuming the PQI is SD rather than SDHC you do NOT need a modified kernel - the stock Nokia kernel should work assuming there is nothing unusual about the PQI card itself.

If the PQI 4GB card is SDHC (aka SD 2.0) then you WILL need the modified kernel referenced in post #73 - without it the N800 won't be able to access the card at all. In order to update the N800 with the modified kernel you need to download the flasher 3.0 utility from maemo.org and run it on a Linux x86 PC (or use a LiveCD distro). Post again if you need more flasher instructions and your proposed card is SDHC/SD 2.0.