An SD-HD card can be larger, but the old "SD" card spec stops at 2GB. If your gizmo says "SD cards" that means 2GB maximum unless you luck out. If the gizmo says "SD-HD" then you should be OK for the larger capacity cards.
Garmin states that Nuvis can take up to 16 gb cards. Whatever you do, do not try to format the card beforehand, particularly if you have a Mac. I tried and the card would not work until I put it in a Canon digital camera. It worked fine after that when I used it with my Nuvi 760.
I am using a Sandisk 16gb Class 4 card... Seems to be just fine since I have almost completely loaded it with music and have used it 4 years now... I am hoping to put in a 32 gb Class 10 card and try it because it will be faster. I truly like the variety of tunes on a long road trip...
Just tried installing a 32 gb HD card in my Zuma 660, loaded it with 20 gb music mp3's and it took forever to find a song and would not even play it. Put back in my 16 gb hd card with 15 gb music , now works great