i read part of your other linked thread...if you are going to be at wdw 21 days you will probably want more than one memory card

( boy am i jealous!! it sounds like you are planning a great trip!)
i take lots more pictures with digital than i did with film just because if you don't like it or it doesn't work out you didn't spend anything for it so who cares? on a 2 week trip( not wdw) i used almost 6 gb total but if you are going to have a laptop you wouldn't need that many cards. but for somethings like fireworks you probably would want to take more than a 1 gb with you since it would be hard to have time to really compose and review with out missing the next shot so I'd make sure you have at least 2-3 gb ( if you are going to be downloading to something every night or so, if not i would double it but then i am neurotic about being prepared) just so you can have plenty for things like that..you'd hate to run out mid Wishes. make sure you have extra batteries also for the same reason.( not because i'm neurotic , because you don't' want to run out) and if you have been using a card all day, make sure you have a bunch of pictures left before something like fireworks start cause it is annoying to end up with a full card you have to change mid what ever event.
check around cause lots of times you can get sd cards for around $15-20 per gb. also i doubt the cameras you are interested in will use the speed of prospeed cards( i know my rebel xt doesn't) so don't go over board gettting something really expensive just because it's tons faster. you might not get the benefit of the speed
also imo it's worth buying a few more cards to use the best setting ( ie the largest jpg) cause it will allow you to work with your pictures more when you get home and still have decent photos...imo that is why i might wait to send them in till after you get home...that way you can look them over, crop out the 52 total strangers that are in the picture with you and the little kid with his finger up his nose that might other wise make the photos "less special" jmho but like i said that's the neurotic factor