What SD card should I be using for my nuvi40?

Not really, but you might like to read this FAQ

https://support.garmin.com/support/searchSupport/case.faces?caseId={18331ce0-4af6-11e3-f27b-000000000000}

In particular note this

Using an SD card size larger than 8GB is not supported for these devices. However, using one will not harm the device but it may or may not work. Also, Garmin supports all speed ratings (including unmarked cards).

found this little stub of a thread fortuitously as it addresses the exact same issue that I have. I too have a nuvi40 and was even using an XP machine (no longer supported) to run updates. I installed the update software on a different machine and tried to do an update and it now complains there's not enough memory on my SD card to update the maps. I dug through all the documentation, etc. once before and found as your response above indicates that 8 GB is the max size allowed, so that's what I bought. Interestingly, the error message on my device during this update said that up to 32 GB is allowed. Further, the page that you reference seems to say the same thing (contrary to your message above). Has a software upgrade removed the 8 GB limit? Of course, the next question, if this is so, is there anyway to transfer all my map sets that I've previously paid for to a new larger SD card required to support the bloating of the map set for which I have unlimited updates (frankly it appears the answer is no, but Garmin is so poor in terms of support that it is hard to tell for sure). Any clarification (or even pointing to where I can get Garmin to answer these questions) would be appreciated. TIA

The maximum recommended used to be 8Gb as posted above, but that has now been changed to 32Gb. You should find that 8Gb though is ample for a nuvi unless you're loading more than one set of maps. However should you wish to, simply copy all the folders/files from the 8Gb card to the 32Gb one.