Is there a huge difference in speed and connectivity for 10 mhz and 20 mhz or even 40 mhz?
All things begin equal, a channel that is twice as wide, will give about twice the speed and capacity. Not quite, but in theory it can be twice as fast.
Of course, if using a larger channel bumps you into more interference, and if a narrower channel fits you into a small clean spot between other interferrers, then in that situation, a narrower channel can in potentially provide more performance - so with Wireless, you have to test it in your environment, since every situation can vary. : )
Each time you halve the bandwidth you halve the potential throughput. 10Mhz max theoretical is a bit more than 50Mbps, 20Mbps 100Mbps and change 40 is twice that (though less likely to be achieved thanks to noise more often than not).
ninedd and I crossed post streams
Throughput provided by the channels is almost totally proportial to the channel Bndwidth.
40 MHz ~ 200 Mbps
20 MHz ~ 100 Mbps
10 MHz ~ 50 Mbps
5 MHz ~ 25 Mbps
Please note channels with smaller Bandwidth are more stable in noisy environment because of higher spectral density.