Ouch! Ten days with 3gb and 8mp? I think you're going to need more (much more if you're anything like me). You could reduce the image size or the quality but then, well, why did you buy such a nice camera? It seems like a waste to lose all that detail and produce mediocre, lo-res, point and shoot quality photos. Figure you'll take 1000-2000 photos in 10 days. So, buy more cards (don't buy them at the park, they're too expensive), figure out a way to transfer to your laptop (do you need a card reader?), buy a portable storage device ($100 and up), or plan to spend something like $12 (?) per CD to archive at the park (that's 640mb).
I personally like have a portable storage device because they start at 20gb and move up from there. That's a cheap way to make sure you don't run out of space. Others like to have gobs of cards. Laptops are good if you already own one but it's just one more thing to have carry through the airport. And archiving to CD at the parks can add up really fast (but I guess it takes up zero space at the airport).
Don't limit yourself and don't hobble the performance of your camera. You're probably spending multiple thousands of dollars already on the trip not to mention the money you spent on your camera gear. Make sure you have every photo that you could possibly want or you'll regret it later.
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