OK; that's a good start. You've got routers that are capable of running the latest IOS releases (if they are maxed out on both RAM and Flash).
Now since neither the 3640 nor the 2611XM have any built-in means of connecting to an ADSL line, so you will need to do one of three things:
1) Buy an ADSL WIC to fit into one of your routers. The card to look for is a WIC-1ADSL= but these can be expensive.
2) Buy a small office/home office (SOHO) Cisco router which has a DSL interface as its WAN interface.
I went down this route, buying a Cisco 827 (I would recommend the 837 or 857 instead, as they are switched on their inside interfaces, not just hubs) - the commands to configure the WAN interface on these small routers is the same as they are on the 2600-series routers (and these can use RIP, so you can advertise your internet route back to your other routers) so you get the same experience.
3) Connect your Cisco router to an ISP-supplied DSL router via 10/100 Ethernet. This is good if you've already got a working internet setup at home, and don't want to mess with it. Troubleshooting connection problems with these is easier as the ISP will typically support the devices they supply (whereas they might not support a Cisco router with DSL interface). Some are more basic than others, but some support SNMP, routing protocols, QoS, etc.
Any of the above will give your Cisco routers a means to connect - you just need to work out the configuration on the routers. Depending on your setup, I (or someone else on these forums) may be able to help you if you have any issues configuring your routers.
Hope this helps!
Kind regards,
Graeme
