The first thing you need to know is that all the classes above 450 are sums of the maximum link rates of each radio. So each radio in an N600 router maxes out at 300 Mbps and at 450 Mbps in an N900 router. AC routers are a bit trickier, since link rates are different for each band. For example, in an AC1750 router, the maximum link rate you'll see with a 2.4 GHz link is 450 Mbps or 1300 Mbps when connected on the 5 GHz band.
The next thing you need to know is that the 2.4 GHz number in Table 1 is 2X the number you are likely to see reported by your device's network status / properties when you connect to your new router. This is because N and AC routers support different maximum link rates depending on the bandwidth mode they are operating in.
The 2.4 GHz link rates shown in Table 1 are the best case when the router is in 40 MHz bandwidth mode. The 5 GHz link rates require 40 MHz bandwidth for N routers and 80 MHz bandwidth for AC types.
The most confusing N type is 750, because each band in the router has a different top link rate and there is no way of knowing which band it is unless you dig into the product's data sheet. And, sadly, even then, it's near impossible to tell for some products. Some products support the maximum 450 Mbps link rate on the 2.4 GHz radio, while others support it on 5 GHz.
To confuse things even further, the rates you may see reported on your Windows notebook might be lower, depending on the chipset your client device uses. Some N adapters will show only maximum link rates of 117 / 270 instead of 130 / 300 and some N150 products may only show a maximum 135 Mbps link rate. This is due to differences allowed by the 802.11 specification and it's all perfectly valid. A full list of 802.11 rates is shown below so you can have fun trying to find the "speed" number your computer is showing.
802.11n/ac MCS table
(courtesy WITS via Aerohive blog)
All AC routers concurrently support both 2.4 and 5 GHz bands. The exceptions are small travel routers with one radio that must be switched between bands.
The last thing you need to know about these link rates is that both the router and device need to support them to achieve the rates shown. If you have an N150 client connected to an N900 router, the highest link rate you're going to see reported by the device is 150 Mbps, no matter what settings you futz with on the router. How Fast Can Your Wi-Fi Go? takes you through all the combinations, so that you can see exactly what you'll get for any combination of client and router.
The rule is: The lowest link rate always wins. But just because one device operates at a less-than-maximum link rate, that doesn't mean all devices do. Routers are perfectly happy supporting each device at all the link rates it is capable of.
All AC routers operate in both 2.4 and 5 GHz bands, so you don't have to agonize over choosing a single or dual-band router anymore. If you still want an N router, but want simultaneous dual-band operation, look at N600, N750 or N900 class products. In eigther case, you'll still have to choose a band for your dual-band enabled devices to use. So here's what you need to know to make that decision.
The 2.4 GHz radio band is shared by many non Wi-Fi devices, including some cordless phones, intercoms, baby monitors and microwave ovens. It's also where most wireless networks have traditionally operated, at least in the U.S..
The more of these devices that are in range of your wireless network, the lower and less consistent your speed will be. In really crowded areas, you may find your laptop or other wireless device constantly dropping connection to your router, or speeds wildly swinging from fast to snail-slow.
The 2.4 GHz band has 11 channels (in the U.S.), each of which is 20 MHz wide. If you do the math (or look at Figure 1) you'll see only three of them (Channels 1, 6 and 11) don't overlap. Channel overlap is bad, because it's another form of interference, which reduces your wireless LAN's speed and reliability.
Although there is nothing stopping you from setting your router to any of the other channels, for best performance, use only channels 1, 6 or 11. Contrary to what you might think, using the other channels doesn't improve performance. That's because your signal looks like interference (noise) to networks on 1, 6 and 11 and vice versa.
Figure 1: 2.4 GHz band channels
Wi-Fi Hotspots: Setting Up Public Wireless Internet Access
(Cisco Press, 2006) by Eric Geier , used by permission
Dual band N and AC routers provide access to the 5 GHz band. The channels in this band are also 20 MHz wide. Figure 2 shows the complete list of channels in the 5 GHz band. Because the frequency range the band is allowed to use is greater than in 2.4 GHz, the channels can be spaced further apart and don't overlap. The lack of overlap means you can set your router to any of the 5 GHz channels your router shows.
Figure 2: 5 GHz band channel list
Because some of the frequencies are shared by public safety services like police radio and radar, only channels 36, 40, 44, 48, 149, 153, 157 and 161 are used by most dual-band routers. (Some routers throw in Channel 165 for good measure.)
The availability of more channels and the relatively low use of 5 GHz was an advantage of 5 GHz vs. 2.4 GHz. But 802.11ac's 80 MHz bandwidth mode eats up four channels, reducing the number of 5 GHz channels effectively to two. And the major transition from N to AC routers means that 5 GHz no longer has tumbleweeds rolling through it.
One piece of good / bad news is the 5 GHz band's reduced range, due to the signal's higher attenuation when passing through walls and other physical obstacles. So a router operating the the 5 GHz band may not be able to provide a usable connection in the same location that it can when using the 2.4 GHz band. On the other hand, since it doesn't go as far, your router's 5 GHz signal is less likely to interfere with your neighbor's and vice versa.
One more thing. Before you decide that 5 GHz will solve all your problems, you should know that many older mobile devices (phones, tablets, e-readers) don't support 5 GHz operation. So you may need to buy a new tablet or smartphone to take full advantage of the higher throughput your AC router can provide. If you don't want to upgrade right away, your devices with N radios will still work fine, since AC routers are backward compatible.