On my digital camera I noticed a performance decrease when I used a larger card; will this happen with the Raspberry Pi too?
What's the maximum sized SD card it can handle?
I see there is comprehensive list available from this question.
See this link for a list of compatible cards:
This is a driver issue and hopefully should be fixed soon, check the list before deciding on what to buy is my advice.
Updated link: http://elinux.org/RPi_SD_cards
Performance shouldn't decrease with larger cards (I'd say the same of your camera, perhaps it's more an issue with too many files/directories - some devices don't handle that well).
The Raspberry Pi supports SDHC cards, which can have capacities up to 32GB.