TL;DR - If you want faster speeds, just find a 2:2/300Mbps capable 802.11n wifi card to drop in to your laptop to replace the 1:1/150Mbps capable one in your laptop. That'll roughly double your speeds, give or take a little.
To quibble Thiggins, every 20MHz uses 4 channels, as each channel is 5MHz wide. The channel you choose is what your network is centered on. So channel 1 actually is centered on channel 1, +/-10Mhz (so it'll cover channels 2 and 3 on the high side). Channel 6 is centered on 6, but it'll overlap in to channels 3, 4, 7 and 8.
OFDM/CD for 11a, g, n and ac specifies that the channel selected is the center and strongest part of the radio broadcast, but the signal strength does and is required to drop off from the center. In the edge, it is required to drop, I think it is, 20dBm by the time it reaches the edge of the frequency band used (which is a thousand times weaker than the center). In effect what you see modeled in diagrams and stuff is a trapozoidal wave form with roughly 15Mhz at maximum strength and the 2.5Mhz or on each side dropping off in a linear fashion to the noise floor. That isn't exactly how it would really look on an oscillascope because radios, amps and filters are not going to be that perfect.
If you had a router or firmware that allowed you to force 40MHz, of course you could use it. That might not be best and it likely would make you a rude wifi neighbor. In my experience in my townhouse where I had a ton of neighboring wifi networks, I could use 40MHz mode and it worked great, in the same room as my router. If I left the room it was actually slower than setting it to 20MHz mode and I am sure it added extra interference for my neighbors networks.
In my current house I have zero networks near me (+1 semi-rural living!), so 40MHz is fine and works great.
One of the things you can look at doing is getting a router that supports 5GHz, even if just 11n, and a client wifi card that supports 5GHz. That will almost deffinitely allow you to use 40MHz mode in 5GHz as well as having minimal to no disruption from neighbors, both because of all of the extra channels as well as the low penetration of 5GHz.
That would represent a rough doubling of your wifi performance by being able to do that, at least close to the router. 5GHz drops off fast, so a few rooms over it might not be much if any better than 2.4GHz at 20MHz is, but if you set it up right, your client will switch to whichever band (2.4 or 5GHz) is going to be faster at any given point in time.
The number of streams is independent of the channel width you utilize.
In general with 11n, 1 stream if 65Mbps, 2 is 130Mbps and 3 is 195Mbps at 20MHz. On 40MHz it is generally 150/300/450Mbps. There are some exceptions, a few products will advertise and link at slightly higher speeds on 20MHz. I've seen some single stream 20MHz that advertises 72Mbps and 2 stream at 144Mbps (I don't think I've seen any 3 stream that advertise higher speeds).
My Netgear 3500L actual advertises 144Mbps on 2:2 20Mhz and I can at least verify that Windows claims that as the link rate if connected to it. I have no way of knowing if there is any performance difference or if there are any tweaks to get there, or if it is just a different way that the router manufacturers advertise what the speeds actually are.
The reason why channel bonding, going from 20MHz to 40MHz and also 40MHz to 80Mhz increases bandwidth by a little more than double is for the reason I mentioned above. OFDM/CD requires that signal strength drops off by a lot once you get near the edges of the frequencies you are utilizing. This means the edges of the frequencies aren't terribly useful for transmitting or receiving data because the radio signal is so much lower. Basically what is transmitted in those edge frequencies has to be done at a lower rate than the central parts. So, if you assume on 20MHz, you can use 15MHz at max speed and 5MHz (2.5MHz on each end) at reduce rates, you add it all up to get your 65Mbps (single stream). Now, if you bond a pair of 20MHz blocks together, you still have to reduce signal power in the outer edges, but now, you have 35MHz at full power and still only the edge 2.5MHz on each side (5MHz total) at reduced power and speed.
That is why a single stream at 40MHz is 2.3x faster than a single stream at 20MHz is, because you have less edge frequency at lower power and speeds as a ratio of the overall carrier.
