cisco 871w router ios upgrade

Hello, I am trying to find out if I can upgrade my cisco 871w router to ios 12.4 but not having much luck with the cisco website. Currently, show version gives:

Version 12.3(8) YI1, RELEASE SOFTWARE (fc1)
Synched to technology version 12.3(10.3)T2.

(1) What is the latest version of ios I can put on my box?
(2) What is the order number?

Anybody had this problem and worked it out I hope?! Thanks! Scott