Hello Mauricio,
to answer your question you should use the feature navigator inside CCO
www.cisco.com/go/fn
use search by feature set
type PPPoE server
specify 1841 as platform
and you get
12.4(21) main release advanced ip services, advanced enterprise services,
that require 192 MB RAM 64 MB flash
advanced security, broadband, enterprise base, enterprise services
that require 128 MB RAM 32 MB flash
Hope to help
Giuseppe