I used to think getting a high capacity memory card is asking for trouble. But with RAW files getting larger (about 12 MB each for EOS 450D/XSi), high capacity memory cards aren't so high capacity anymore. Furthermore, it can be challenging to change the memory card in some situation. And due to tiny size, it's all too easy to drop or misplace SD/SDHC memory cards.
If you care about keeping your photos safe, always bring a backup solution. Plan on backing up photos each night, either via notebook or portable media backup solution (e.g., Epson P-3000, 5/5.5G iPod with iPod Camera Connector). At $250 after mail-in-rebates, 40 GB P-3000 is a pretty decent value.
Estimate the average number of photos you take before they can be backed up, and multiply it by the average file size. That said, it is a good idea to bring another memory card in case the first one fails.
As for the performance, just stick with class 4 or higher from major manufacturer (e.g., Lexar, SanDisk). Extreme III is supposed to be more weather resistant than Ultra II if that means anything.