Just got back, so here's my thoughts!
The wonderful thing about digital is that you can erase bad photos right on the spot, therefore they don't take up space on your cards. I took 2 256MB cards and shot all my photos on the highest megapixel setting and I didn't even fill up one card because I deleted blurry, bad photos.
You could always purchase an additional card (keep it in original packaging) and if you don't need it, return it since it will be unopened. Or if you end up needing it, at least you have one with you instead of paying outrageous prices at the parks.