Count on an average of about 4 Megs per highest-quality JPEG. That is an average, of course. Scenes with more detail, such as a picture out in the woods full of trees, will take up to 7 or 8 MB each, depending.

If you shoot RAW, count on about 8-10 MB a picture.

I have a Digital Rebel (300D - 6 MP) and carry six 512MB cards. I usually get around 140 high quality JPEGs or about 70-75 RAW images on a card. Scale your results by about 3/4 (seeing as how your images will be about 4/3rd the size of mine.