On my recent trip to HI, I shot RAW+embedded Large/Fine JPEG and the files from my 6.3MP camera ranged from 8-14MB, averaging around 10. But unless you want to deal with a digital darkroom, RAW converters, custom white balance, gray cards/WhiBal, color spaces, etc., I wouldn't recommend it.

Just shooting high resolution JPEG averages about 2.8MB, with a range from 1.5 to about 3.8. This number should be directly proportional to the MP within the camera sensor, assuming each camera vendor implements the JPEG compression alogorithms in a similar manner.

Shooting lower resolution can save you space but, as the article said, CF cards cost me about $110 for 2GB just before I left, disk is about $0.25/GB, and CD's can be found free after rebate, so saving storage isn't much of a deal. And finding that you undersized that one magic shot that you want to enlarge would be a bummer.

Dpreview.com for FT-equivalent in photography. There are lots of other good sites, but that's where I tell folks to start.