it depends on the router and what you are trying to accomplish. most routers are backwards compatible by at least one generation, so I would assume at least some or most ac routers also support 802.11n. If that is the case you can connect any of your current N devices, but not at AC speeds. to get AC speeds you also would need an AC capable receiver in the client device.
- Im not a pro but hardwired devices will not see any major improvements. ( its always possible a newer router may handle the traffic a little more efficiently)