How many mega pixels is your camera? If you're using a 4MP camera and set it at the highest resoultion, you'll get approximately 105 pics on a 256 Mb card (I think yours uses SD memory)
For my trip in February, I took about 190 pics in 5 days. However, that was the first trip to WDW I had with my daughter, so I'm guessing that 30 pics per day should be sufficient if you've been to WDW many times).
Should you have the choice, get the 12x media or faster so you can take your next picture sooner (if you're into sequence photography).
Get brand-name cards such as Viking, San Disk, Panasonic. The only brand-name that I've had bad experiences are Memorex and Toshiba.
As a point of reference, I use Compact Flash and Secure Digital cards exclusively. I never used anything else other than those two for digital cameras.