I've got a 16GB SDHC card in my 500, and it works well, if a little slow to search the music and radio plays that I have filed on it.
The make shouldn't matter, but I would recommend getting one that's as fast as possible if you're going to store a lot of music (the max is 1000 tracks).
One tip - if you have a lot of music, breaking the files up into directories, e.g. for each album, makes the indexing work a lot faster for some reason.
You can store new maps in the unit by creating a Garmin directory on the SD card and saving the maps to that from Mapsource, via the PC. What maps do you want to store - the 550 map already holds all of Europe, or are you off to somewhere interesting?
By the way, I'm not sure, but you may need to update the firmware to get the Zumo to read SDHC cards. The reason I'm not sure is that I may well have filed my MP3s into directories at the same time as I updated the firmware.