The IOS image will depend on the features that you want, for example, to get the full features of IPv6 routing protocols like OSPFv3, ISISv3, RIPng, BGP-4+.
12.3(6a)
12.2(15)T9
Ranging from features sets depending on what version you want to run
ENTERPRISE BASIC
IP PLUS
IP/FW/IDS PLUS IPSEC 3DES BASIC NO VOICE
IP/FW/IDS PLUS IPSEC 3DES BASIC NO ATM
IP/H323 PLUS BASIC
IP/IPX/FW/IDS PLUS BASIC
Please go to http://tools.cisco.com/Support/Fusion/FusionHome.do & use Find software with the features I need
On a 12.3.6a images require 96 Mb ram and 32 Mb flash for most of the images mentioned below.
ENTERPRISE PLUS c3640-js-mz.12.3-6a
ENTERPRISE PLUS IPSEC 3DES c3640-jk9s-mz.12.3-6a
ENTERPRISE PLUS/H323 MCM c3640-jsx-mz.12.3-6a
ENTERPRISE/FW/IDS PLUS IPSEC 3DES c3640-jk9o3s-mz.12.3-6a
ENTERPRISE/SNASW PLUS c3640-a3js-mz.12.3-6a
ENTERPRISE/SNASW PLUS IPSEC 3DES c3640-a3jk9s-mz.12.3-6a
IP PLUS c3640-is-mz.12.3-6a
IP PLUS IPSEC 3DES c3640-ik9s-mz.12.3-6a
IP/FW/IDS PLUS IPSEC 3DES c3640-ik9o3s-mz.12.3-6a
IP/IPX/APPLETALK PLUS c3640-bins-mz.12.3-6a
IP/IPX/APPLETALK PLUS FW/IDS c3640-bino3s-mz.12.3-6a
Thanks, Mak