Remember, it's not so much the screen size as it is the resolution.
The 12" iBook (as well as the 14") has a resolution of 1024x768.
The 15" PB has a resolution of 1280x854. Roughly 40% more pixels.
However, many games look just fine at 1024x768. And fewer pixels means less work for the computer and a better frame rate.
However, the 15" screen has 56% more area (9"x12" as opposed to 7.2"x9.6"), so its pixels are a bit bigger (40% more pixels, 56% more area = more area per pixel), hence a bit more readable. But a LOT more money.
Edit: your 17" iMac has a 1440x900 resolution, 65% more pixels than the 12" iBook. However, note that you can connect the iBook to an external monitor (via a hack that lets you run it at a higher resolution).
However, the iBook isn't exactly a graphics powerhouse, and that, not the screen resolution, might lead you to maybe be a bit disappointed, depending on which games you play. If you can, try an iBook out at a local Apple Store or other Apple retailer.