I use a class 4 card (8GB) and have no problems with music whatsoever. It plays perfectly. I don’t do pictures or video on that tiny screen so I can’t speak for those. Personally, I don’t think paying more for the class 6 is worth it if you’re just doing music. At least that’s the case for WMA and MP3. I don’t know if it’ll affect OGG or FLAC since support for those formats hasn’t been added yet.
That’s an interesting question about 16GB card, though. When I bought a 4GB (PRO Duo, not MicroSDHC) card for my Sony PSP (gaming/MP3/video device) I noticed it definitely seemed slower than the 2GB card I had used previously, despite them both being Sandisk cards of the same type, differing only in capacity. I hear the 8GB cards are even slower yet and not suitable for some purposes, though I’ve never felt a need to get one that big myself.
Of course, the PRO Duo and MicroSDHC cards are very different, and the Fuze and PSP have different operating systems so these experiences might not be comparable. In fact, I’ve noticed that my 8GB MicroSDHC card takes the same amount of time to load (30 minutes) as my older 2GB MicroSD card on my E280 did, even though it’s four times bigger. So the data I can contribute is kind of all over the place. Probably your question about the 16GB cards can’t be answered until they’ve been out for a while and people have had time to play with them.
Well, that’s my own input. Doubt it helps much but ya never know…