The main problem I have with 802.11b is the lack of range. There is a tremendous difference between my iBook's 802.11g and my dad's Thinkpad's 802.11b. I will never have trouble with a signal, whereas he never has as strong a signal as me, and sometimes no signal at all (though this is probably windows-related). Anyway, under the right conditions, b is just fine for internet. As far as routers, some g routers will turn into dogs when a "b" machine is connected, while others are just fine. For example, in my apartment, we have a Netgear 'g" wireless router that is great when it is only me and my one roomate using our "g" cards. However, when a "b" card is connected, I notice a drop when I'm downloading large files. Basically, just go to cNet or something like that and read reviews of routers if this sounds like it would be an issue for you.
btw, the previous posts are correct about the timing of airport cards. If I remember right:
Powerbook G3 Pismos, Titanium Powerbook G4s and all iBooks G3s had Airport (802.11b). Aluminum Powerbook G4s, iBook G4s, and probably all intel portables for the remainder of 2006 will have Airport Extreme (802.11g). Expect Apple to intro Airport ______ (802.11n) sometime between October 2006 and January 2007.