first of all: you won't be able to find airport extreme for the ibook. just good old airport using 802.11b which is 11Mbit/sec.
And it won't be any prob to plug your adsl modem into your airport extreme base station. The thing about the 56k is, that your base station also has a plug for analog lines, so you can use your ibook wireless and enter the internet over the 56k modem. But since you got adsl, you won't be interested to this...