It depends on what feature(s) you want to utilize (PPPoA/PPPoE, PPPoEoE/PPPoEoVLAN, RBE (Route Bridged Encapsulation)RFC-1483, PPPoA VPI/VCI accounting, PPPoA/PPPoE autodetect, L2TP, etc). Some of these are supported on the 3660 platform, but your best bet is to check the feature navigator to see what platforms support the feature you are interested in:
http://www.cisco.com/cgi-bin/Support/FeatureNav/FN.pl