Core Duo MacBook specs (circa 2005-2006):
Intel CoreDuo (not Core2Duo) dual core processor @ 1.8-2.0 Ghz
Max RAM 2 GB (uses DDR2 PC 5300 200 pin DIMMs)
Besides the fact that the iBook will run up to Mac OS 10.5, and the MacBook will run 10.6, you have speed differences. The Macbook is roughly 4-5 times faster than the iBook. The screen on the MacBook is slightly larger (13.3" diagonal vs. 12") and at a higher resolution. The MacBook uses and internal SATA 2.5" hard drive, while the G3 uses a PATA 2.5" hard drive, and has issues recognizing drives larger than 128 GB (chipset controller limitation, not OS related).
The iBook uses USB 1.1, while the MacBook has a USB 2.0 chipset. Both have FireWire. Both can connect to an external display, though the iBook only supports mirroring on the external display (though someone wrote a kext hack in OS 10.3 that allowed you to span your display).