I don't have MP3s on my Kindle. I don't listen to music while reading and randomly playing the tracks in an audio book sounds like an interesting experiment, but not a very satisfactory way to follow the plot.
I have a 2G SD card in my Kindle -- taken from my camera when I upgraded it to 8G so it would hold a decent-length video. I only have 100 book files on my Kindle (core memory and SD card combined) because I don't want to have to search through too many books. Instead, I keep my entire eBook library on a USB drive that lives in my backpack and goes everywhere with me. I can mount the USB drive and the Kindle at the same time and move files between the two very easily.
When my sister got her Kindle, I bought a 1G SD card for $14 and put over 250 files on it -- many files containing multiple books. Those files took up only 135M of the 1G.
As you can see, the way I use my Kindle, I'm not a candidate for a large-capacity SD card. Now, give me a software update that provides some kind of organization by folders or "bookshelves" and gives me MP3 playlist capability -- I could change my mind very quickly.
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