I like the 8GB card I have, it is class 4. I opted for the additional storage, because my 8GB card content remains static. I don’t sync to it. I load it once with my favorites and forget it.
I actively sync using intenal memory. I use 2GB for Rhapsody Channels, and Actively Refresh the other 6GB with Winamp. The 8 GB card holds all my 80’s music (2GB) and 6GB of my favorite albums (a mix of genres and artists).
For me the 8GB card was the way to go, since I only have to load it once. (Took about 40 minutes in Drag and Drop MSC mode)