Since there can be only one SD memory card, I decided to get the largest size possible. The Kindle is compliant with the SDHC High Capacity model (even though they don't mention it, and their suppor center did't know it until I told them), I got an 8 GB SDHC memory card which shows 7.6 GB capacity on the Kindle, and works just fine.
I recognize the need for a better Directure structure, but you take what you can get. Hopefully by the time mine becomes too difficul to use, they will have come up with a better structure.
Since I plan to put a lot of audio books on my Kindle, and these take a lot more memory than an ebook, I really wanted the additional space a memory card affords to my original memory capacity which was only 181 MB.
All in all, I really love my Kindle and can't imagine living wihout it.
Charles Wilkes, San Jose, Calif.