a 1gb card should hold around 260 photos in Large, and around 115 RAW on a 350d
so it just depends on how many photos you plan to take in one day without emptying the card
don't buy cards larger than around 2-4gb,
i'd rather have 2x 2gb cards, than 1x 4gb card, if the card fails, you've lost a lot of data, so its better to have it spread across more cards