If you were to get an xD card, any size could work, it just depends how many photos you take at one time. Get at least a 128 MB, that'll hold around 65-80 photos. A decent size would be 512 MB, which is great if you go on vacation and don't run out of room.
depends ofcourse how much you take photos, and qith what quality. If you are going to travel for a month or something I'd suggest to buy a BIG card, maybe 1gb or even 2
i think your best bet is to go on ebay and buy a 2gb memory card. Thats what I did for a Sony Pro Duo. I got a highspeed one with shipping for 60 dollars "Buy it Now". ( i hope im not breaking any rules about advertising or anything... )
I would go with about four 512 mb cards, that way if you happen to have an error or corruption, you wouldn't lose as many pictures as you would with a 1 or 2 mb card. I know from personal experience, batteries went dead while saving a picture and caused my card to become corrupt and it cost me to get my pictures recovered per picture. HTH,.. George