re: my new ASUS RT-AC56U came in today..damn fast shipping from the Egg

Might as well go for that, then. As I said in the other thread, it's not going to perform like much of an AC router even with AC clients, unless you're within a close range. However, all of the N900s worth having are over $100, and the N600s are hovering in the $70-80 range. The newer, super-duper-powerful AC routers with 4x4 MU-MIMO should push the price of the lesser AC routers down to a level competitive with N900s. Eventually, deals like the one for the ASUS will be more common, and it'll make sense to get an AC router whether you have the adapters for it or not.

It will be another year or two before every new wifi-enabled device you buy will have 802.11ac, and it still only matters if you're using those devices to access files on your local network. If everything connected to your router is just fighting over your broadband connection, then your current router might be overkill as it is. The main concern would just be range, or far can your weakest device stray from the router before its throughput drops noticeably below your available broadband speed.