iRex iLiad:
MMC/SD: Limit 1GB
CF: No limitation
USB Host:
Not for connecting to a PC, but hosting a harddisk or using a Memory Stick. When travelling, in the beginning I sometimes have had a harddisk > 30 GB of comics.
iRex 1000S:
SD: No limitation
When purchasing, 1GB already is included.
Sony 505 and Sony 700:
SD: No limitation
Memory Stick: No limitation
Kindle 2 and Kindle DX:
NO expansion slots.
I guess the idea is, you can download your content anytime via Whispernet. Personally, I don't like that. You may be out of range of Whispernet, or you simply might want to carry 3rd party content with you.
Cybook Gen3:
SD: Limit 2GB
BeBook:
SD: Limit 1GB