I have this machine. They're a few card readers that are compatible with the machine. PED, Ultimate Box, Magic Box or Amazing Box... picking which one you want, depends on the computer/laptop you have, budget and storage.
I went with the Ultimate plus box as it connects to my laptop via USB versus serial port connection which would be for a PC of a lower version. It is the top tier as in price but you won't need an upgrade with this card reader. The most after that is a backup card reader, just in case your first reader dies.
The Ultimate card holds as much storage is on the card verses the other where you can only have 6 designs per card.