My question: Are there are any restrictions, limitations or modifications needed to my using a Wireless-G USB adapter/antenna that complies with IEEE 802.11g and IEEE 802.11b standards on my iBook?
It shouldn't hurt your speed. 802.11g is 54 Mbps, and even 802.11n maxes out at 300 Mbps. You have USB 2, which has a maximum throughput of 480 Mbps. So, I don't know why you should expect a speed reduction. If anything, you should expect a speed increase.
Still, you're going to have to risk purchasing the device and trying it, or at least Googling the individual devices you want to consider.