I have a 128mb card. Ive got 9 banks on it full and still have a little under an hour of recording time. good stuff. I don't like transfering from a computer onto a card, as it takes way too much time to get the samples from the card onto the sampler's banks.
128 mb on SP505 is way enough for me producing hiphop beats. If you are a DJ using it live then you may prefer better 404 or the new 555 which uses 2GB CF cards.
32 mb , and always have a couple seconds left when the banks are full ive filled it a good 4 times, now that i have mpc 2500 i can quickly backup the waves and start over.