The ASUS RT-AC66U, RT-N65U and RT-N66U routers doesn't have a dsl modem, so thy can't be connected directly to a DSL line. You could connect another DSL router to the line and behind the ethernet LAN port of the first router, you can connect the RT-N66U. This option could be even better if your first router can be used as just a bridge ADSL.
My recomendation, however, is that you evaluate to use the ASUS DSL-N55U as a better option. It is almost as powerful as the RT-N65U & RT-N66U routers, but cheaper and you would also save energy with only one device. When you get fiber to your home, you can sell it and study your new options. This is what I have done since there will be no fiber to my home at least until next year if not 2014.
You should consider some other things before to decide. If you decide by the DSL-N55U router, you should realise that there are two models: Annex A and Annex B. Depending of where you live and the provider, one will work and the other one won't.
If you need the AiCloud feature, it isn't implemented in the last version of AsusWRT for DSL-N55U. I have read in the Asus support forums that they are going to add this feature. It even was expected to appear in february-2013, but as far as I know the new beta release still doesn't implement it and we are in april.
If you want this router becouse you are planning to change the firmware to AsusWRT-Merlin (
http://www.mediafire.com/asuswrt-merlin/), it won't work on DSL-N55U. This firmware is just available for RT-AC66U, RT-N65U and RT-N66U. There is some people working on an alternative for the DSL-N55U that is called Asuswrt-bender, which is ais spin-off of AsusWRT-Merlin, but I think it is not working yet.