I have the Garmin Oregon 550 and I first used an 8gig microsd chip, then when the Birdseye satellite maps came out I upgraded to a Kingston 16 GB - Class 10 - microSDHC and this works great, I don't think the extra speed helps when the card is in the GPS but if I have a big transfer, I will plug the card into my computers card reader and the transfers are many times faster. Then I filled that one up and instead of having multiple chips to swap out I thought what the hell lets try the
Kingston Technology 32GB microSDHC (Class 4). It seems to work just fine, I have 17 gb of images on it right now. I think the Oregon 550 and newer GPS's could be the only ones that will work with these higher faster capacity cards.