I'm looking to get the Nikon D5000, which is 12.3 megapixals?

How much photos will i be able to hold with that much picture quality in an SD card.
Ex. 2GB=?
8GB=?
16GB=?

Cards: anything above 2gb is classified as an SD HC (high capacity) card. Your best bet is an
8gb, as I see it. Take a shot or two, upload, look at the properties and do the math and you'll have
an idea. It should be many hundred photos.

Download the users manual from Nikon's web site. Look on page 215. There's a chart which gives approximations for all the different format options.

For JPEG, fine, large; about 550 shots per 4GB.

I can take about 1200 shots with my 8G card. My camera is a canon G10 14.7 mp. All my shots are shot at High resolution.