Yep, sounds like it has to do with the calibration of your TV (specifically, adjusting overscan/underscan). As it was said, you can adjust the size and position of your screen by going into the service mode of your TV (not sure how to get there on Pioneer's). If you don't do some research and know what you're doing, you could possibly do more harm than good to your TV. You might be better off just living with it.
Are the black bars not noticable on ANY Xbox game? Most of the Xbox games I play don't have them, however, i've noticed Burnout 3 does (Toshiba 57"). PS2 and GC also vary, but you can usually see them.
BTW, composite cables for the GC?

I'd say the upgrade to component (if your cube has the digital out) is very well worth it, or, at the very least, S-video.