it's common knowledge that SD cards are flash chips in MLC configuration.
This makes them fast to read,reasonable to write for single files.
Usually SD cards get used in photo camera's where the device is either in read mode (to digitally view the photo's on the card), or in write mode.
writing orreading a single file, using single threaded reads.
Once you start reading randomly,or writing randomly;and especially when you start reading AND writing randomly at the same time,with more than 1 thread at the same time, the speed of an SD card easely can drop drastically,in the likes of 200x slower.
meaning a SD card with 20MB/s read, 12MB/s write can in such cases drop to less than 0,1MB/s data transfer speed.
In extreme case I had a sandisk rated class 6 (12/8MB/s) which performed under total random read/write multi-threaded stress only 10kb/s
see my test results here:
http://wiki.eeeuser.com/advancedbenchmarks
I would suggest you find out if the PRS-500 with it's firmware update can read SDHC cards,and buy a cheap 4 or 8GB SDHC card if it's supported.
Sandisk Extreme III,Adata,or Topram have many good and reasonable fast SD cards!
There's a reason Sandisk standard SD/SDHC cards are slower. They are most compatible (I yet have to see a sandisk card that can't function on a device supporting these cards),however they are not responding as fast as other cards.
I can not speak for MMc cards of sony. I have no experience with them.