When I got my A920 I was asking the same questions as the handbook states that the phone can take a '128mb' card. Apparently, someone replied saying that it was the largest available card at the time of printing the booklet and the phone will support any size card that comes out. Another chap said he was going to buy the 1 gig card as soon as it came out.
Im running a 256 with no problems- (sd make)
I have the Viking 1gb MMC SD card in the A920, works a treat, got 12 hours of music on the card at the mo, Also managed to use the MP4 converter to size down a film and fitted it onto the card for a long train journy. Very good