hi,
the data rate 11Mbps is depending on the Wireless card. if you have an 802.11b then then it is working as designed if data rate is showing 11Mbps. See SUMMARY in this:
http://en.wikipedia.org/wiki/IEEE_802.11
if you want to do a replacement, get hold of the hardware maintenance manual from IBM/Lenovo website for your model of T42. this will guide you in doing the removal and replacement.
the issue you will have (IMO) is that IBM BIOS does not accept just 'any' card. i read somewhere that it only accepts certain WiFi cards. if your replacement card is NOT on the accepted list, you may get an 1802 error or Unauthorized network card error.