On my 20D, with a 2 Gb card, I can get nearly 175 RAW/large JPEG pairs at iso 400 (initially, the camera estimates 147 pairs at iso 400, but it always allows more, and if you drop the iso to 200 or 100, you get even more images on the card). The RAW/large JPEG is the combination I shoot now for nature. JPEG because it's fast and easy to process if I need it quickly, RAW for when I need its capabilities for detail and big enlargements. I have two 1 Gb cards as well, for a 4 Gb total. Most of my work is day trips into the wilderness within 30 miles of home, so memory hasn't been a big issue. If I were traveling more for overnight shoots, I'd definitely have to pick up a portable hard drive/CD burner or a laptop, because 4 Gb wouldn't be enough for a multi-day shoot.
Doing the math, 4 Gb in cards gets me about the equivalent of 10 rolls of 36-exposure film, though of course being able to weed out the unacceptable images on the digital camera makes that comparison less important.
Another good book for RAW shooters is Rob Sheppard's "Adobe Camera Raw: For Digital Photographers Only." Step-by-step, easy to understand RAW processing procedure to make those images sing. It helped me quite a lot.