Im using an 8800, and i have an 8gig in mine, its using all 8gigs of it too.
first thing i had to do to get it to work was to get it hooked up in my computer, you can goto amazon.com or buy.com and see the exact 8gig card i have, it comes with this little black and red thing that looks kinda like a flattened whistle, there around 50 bucks now for the usb drive(the flattened whistle thing) and the 8gig card. So i put the 8gig SD card in that, put it in the comp, format it. then put whatever files on it i want and put it back in the phone.
Doing that works like a charm, but if i ever take the card out and put it back in, then on average, i get an error once a day saying there's problems with the media card, but it works perfectly fine, every bit of it. Now i just use the desktop manager software to put anything else on it that i need.