What memory card should I use?

Sorry if this is in the wrong section!

I am buying a Canon G9 tomorrow, and I am wondering what memory card is the best?
SD I'm assuming, but how many gigs, and all that type of stuff. I'm not sure how stuff like that works, to be honest. :]
I want good quality, but I don't want to spend too much money, but I also want quite a bit of space on it (over 500 photos, if possible.)

I have NO idea how many gigs hold that many pictures, or anything like that, so any suggestions would be great! Also, I'm not sure if there are any that can't record videos, but I would like it if it could have videos on it too.

I am going to be using this camera underwater, if that matters at all. I will also be using it out of water, of course, but it's main purpose is u/w.

Thanks a lot!

2 GB will be ample for shooting at the highest resolution JPEG with minimal compression. If you decide to shoot a lot of videos or use RAW (which means you'll have to post-process everything) you'll want a 4 MB card, or maybe a couple of them. But time for that as you get experience with it.

Personally I would choose a 4GB SD Card, for it will give you ample space for photos and some movies.