Card is device dependent
I have a gift card for Amazon, thinking of buying the lifetime updates for my 350 nuvi.
Any thoughts on the matter and was wondering how big of a SD card should i buy for the upgrade, or if I need to buy a SD card.
If all you are doing is upgrading your maps you don't need a SD card. You can buy the lifetime updates and when you do so you'll get a registration number that is maintained by Garmin. I just updated my Nuvi 760 and it connected to my computer, I ran the software update and it automagically sync'd my Nuvi with the Garmin servers, verified I was eligible for the newer maps, and installed them. Be forewarned that it takes a long time to do an update. I had it running while watching a movie and it took all of two hours. The maps are large, so takes a while to download, and as long to install.
As for a SD card, it is used for about everything else except the maps. pictures (must be jpeg), music (must be MP3), audio books (from audible.com), etc. The Nuvi series can take up to a 16gig card. I would recommend buying a 16gig card, you'll fill it up fast. How long it take to copy music to it depends on how you have it connected to your computer and/or if you are having to convert the codec (say from wav to mp3) when copying to the card.
Here is the link to what size SD cards are supported by Garmin devices;
https://support.garmin.com/support/searchSupport/case.faces?caseId={a43e2650-1794-11e0-7efa-000000000000}