I have a ton of books loaded onto the 64mb card version. I could go on vacation for about 6 months and not need to worry about finding something else to read.
Card version IMO is better as you can then use your card reader (if it supports the format) for quicker loading / unloading of a ton of books.
Normally IMPServe is fine, but if I'm moving an entire series of books (Just purchased waaaaay to many on a fictionwise sale) I prefer to pop the card into the PC and copy that way.