A little Wi-Fi 101 first.
802.11 is the IEEE standard. It comes in A (5GHz 54 Mbit/s), B (2.4GHZ 11 Mbit/s), G (2.4GHz 22Mbit/s) and N (can be a mix of 2.4GHz and 5GHz and speeds of 54-600 Mbit/s plus aggregation of channels or more antennas [MIMO] to give theoretically N150, N300, N600, N750, 900 speeds etc).
2.4GHZ band suffers from overcrowding and interference from other devices operating in the range including microwave ovens, Bluetooth devices, baby monitors, cordless telephones, garage door openers and some amateur radio equipment.
5GHz band is less crowded but not as good at penetrating walls and solid objects in its path so in practice has reduced range to 2.4GHz.
Each band has a number of channels and that is important for avoiding some interference.
No matter what the claimed Mbit/s speed is it will be significantly slower in practice i.e. G can achieve about 22Mbit/s in reality.
AC is the standard under development that uses 5GHz and multi-station WLAN throughput of at least 1 Gbit/s using single 500 Mbit/s links. Initial offerings will be single [channel] stream (450Mbps), Dual (900) and three (1300).
AC will be backwards compatible with N class clients (receivers) because they will also have an N class router chip in them but to get maximum throughput you will need to use AC class receivers (PCI-E and USB) and not mix the environment. We will start to see AC adaptors in notebooks this year.
Current products include (model numbers subject to change)
Buffalo AirStation AC1300 N900
NetGear R6300 (claimed 1750 Mbps)
Belkin AC1200 DB
Linksys EA6500 (note they have just purchased Linksys from Cisco so look at some model/name changes). See Stephen Withers iTWire article here
Speed, speed, speed is all that matters. There is an insatiable want to stream video around the home and internet in every nook and cranny. So if you are looking to buy a new router it is safe to buy an AC. Sure you will pay a small premium but at worst you will get an excellent N class router.
Note however that speeds are theoretical and frankly if they can maintain say better than 50Mbit/s then that all you need for streaming video.