The iPhone 3GS took photos at 3MP. Each photo took about 1.3MB. The new iPhone 4 takes photos at 5MP which will increase the size to about 2.7MB per photo.
The iPhone 3GS records 640x480 video at 30 frames per second. Using a compression ration of 40:1, When Will Apple found that 250 mins of iPhone 3GS video recording took 9.66 GB of space.
Using a recording resolution of 960x720 at 30 frames per second, When Will Apple calculates that the iPhone 4 will use 21.73 GB for 250 mins of video.
If resolution is 1280x720, iPhone 4 will use 28.97GB's of video (118/min)
It's clear to see that if you plan on taking a lot of video, you should definitely purchase the 32GB iPhone 4.
Hit the link below to see how these numbers were calculated.