Does moving my router from 20MHz to 40 MHz increase my wireless speed?
From Linksys's website:
NOTE: The wireless network modes for a Linksys Dual-Band router will vary depending on the frequency band(s) you choose to enable. In the 2.4 GHz frequency, the Wi-Fi signal range is divided into channels each at 5 MHz interval. Adjacent channels overlap and will interfere with each other at 20 MHz block. Setting the channel width to 40 MHz network will allow you to use 2/3 of the entire Wi-Fi band. Thus having a higher chance of overlapping and interfering with other wireless networks. Meanwhile, if you set the channel width to 20 MHz, the network will only overlap with the two channels before and after that frequency.
You won't get better speed by doing that change. You may get better signal, but there's a downside to it. Essentially, you will have a higher chance to have collisions with other wireless networks around you. I would keep it at 20 just so you have less packet loss.
Dual-band routers essentially give you two access points with each having their own bandwidth in them. Usually one AP will be in the 2.4GHz range and the other will be in the 5.0GHz range. Within each spectrum, there are several Wi-Fi modes that you can enable. The fastest will be Wireless N, with speeds of 300Mbps. However, that 300Mbps is shared between all devices connected to that AP.
For example, if you have 5 devices in the 5GHz AP and one is using up 200Mbps, then the other 4 devices on the 5GHz AP will have 100Mbps to work with.
However, both AP's are isolated from each other (wirelessly anyways; they have the same IP address so devices on different APs can still contact each other), so if you have two data-hungry wireless devices you could put one in the 2.4GHz AP and another in the 5.0GHz AP(assuming it supports 5GHz Wi-Fi).
If you want to get the fastest speed out of your router, enable both the 2.4 and the 5.0 APs, configure them with wireless N, and split your devices across each network evenly one by one.
I just want to point out a misconception about "splitting your devices evenly across networks" that is propagated in the accepted answer. Under normal usage you should absolutely not do that.
What you should do is simply connect all your "speed-hungry" devices (laptop, tablet, smartphone) to 5 GHz network. It is a faster network with lesser interference. The more devices connected to it → the better for them.
Put all other — not "speed-hungry" — devices (like printer, 'smart-toothbrush', Nest thermostat or whatnot nowadays) on a slower 2.4 GHz network. Don't worry, they will be able to communicate with 5GHz-devices just fine, but keeping them off 5GHz network might help preserve high speeds and low latency for those devices that actually need it.
Nevertheless, the fast 5GHz network has limited range and penetration capabilities, so at some point in your house you will probably notice that devices have trouble connecting to it. This is exactly the point when 2.4 GHz network comes in handy once more, because having good signal on a slightly slower network is always better than barely being able to connect to the fast one.
Yes. If your NIC supports channel bonding, setting up your wireless access point's channel width to 40Mhz will double your throughput as you get double the connection speed.
Depending on your wireless NIC you can get up to 300Mbps, or even theoretically up to 600Mbps, and this means you have a higher real speed.
But at 40MHz you will have a bigger chance for channel overlapping and thus interfering with other wireless devices.