802.11b's Max (theoretical) is 11Mbps, but that doesn't account for all packet overhead.... so in reality you won't see more than 5Mbps in ideal situations. The average being more 3.5-4Mbps.
Many broadband providers now offer 4-8Mbps (I know Comcast's basic tier is about 6Mbps with bursting up to about 10Mbps).
802.11g will offer you 54Mbps, theoretical... reality is ~20Mbps (with WPA encryption).
802.11g should be more than enough for a home WiFi install for at least another year, and likely 2-3.
Because broadband providers lack the upstream performance (due to technical issues, as well as policy to limit the impact of P2P and viruses impacting network performance)... you won't feel much more of an increase in downstream performance. At least not enough to pay for (meaning not enough for providers to offer until they upgrade networks significantly).
I really hate how some companies pitch the idea that Gigabit ethernet enhances broadband computing for home users... it doesn't. 802.11g does *NOW* a little bit (though 802.11b is perfectly fine for most). And was pitched as a "giant" performance gain.
Yes, this is just corporate marketing, and I understand why they do it... but IMHO it's dishonest and unethical. If you have 802.11b, don't worry about it. For web *browsing* there is no real performance increase from 1.5Mbps -> 6Mbps because page rendering time is most of the load. Your browser doesn't render 6Mbps worth of data, at least not any computer on the market today. Only file downloads increase in size. Unless you download lots of stuff, or visit very graphic intensive sites... your wasting cash.
I really feel bad when people fall for some of this marketing stuff. Your T41, T42 is still a great computer despite T60's now existing. It's perfectly capable of doing just about anything.
So don't worry about it!

T43 (2687-DUU) - 1.86GHz, 1.5GB RAM, 100GB 5400 (non IBM-firmware Hitachi 5k100) HD, Fingerprint Scanner, 802.11abg/Bluetooth, ATI x300