Got a PRS-650 and tested it. First time I inserted my 16gb SD card straight from my 505 with 230 books it froze
After connecting the 650 to my computer once, it now takes about 4 seconds to scan the SD card, much faster than my 505
Will test later with more books.
As an aside, I do not like the feel of the Sony lighted cover, compared to the trifold my 505 has. (It makes hitting the page turn buttons feel awkward)