The wireless will become the bottleneck in your network when it's the slowest point in your connection. There are, in the main, 2 usage cases to consider:
LAN connections - i.e. if you are copying between devices on your network.
In this case the Wireless connection will almost always be the bottleneck
at the kinds of speeds you are talking about as computer internals are
ISP/WAN connections. If your ISP connection is faster than the actual
speed you are getting through the wireless connection, then the Wi-Fi
connection will be the bottleneck. (On a 20 mbit per second
connection, provided your wireless signal is strong and there is
no interference, the ISP interface will be the bottleneck - However that is not always the case).
1. The link rate for Wi-Fi bears very little relationship to anything - it is a theoretical value that can only be achieved in extremely controlled conditions over short distances - you will not see it in real life. Figures vary greatly, but 50% of the maximum performance is probably about as good as it gets most of the time.
The thing with Wi-Fi is that the further the devices are, the slower the connection. Similarly the more devices sharing the connection the slower it is, AND, IMPORTANTLY, OTHER NETWORKS AND DEVICES in the same (and nearby) frequency bands will decrease your performance. Also the size and direction of your antenna(s) - in the case of your Belkin router this is a big concern, as well as the firmware.
Also, if you use older standards you will get lower throughput, which is where the 54Mbp and 11Mbps comes in - they were the maximums for older technologies (802.11b and 802.11g). Your router obviously supports 802.11n as well as the older, slower standards, which is pretty standard. [If your computer is 802.11n - which it most likely is, you can ignore everything about 54mbps and 11mbps on the box.]
2. If your wireless signal is strong - better than about 40 mbits connection
speed, without too much interference - and this is not a big ask - your
Wi-Fi will not be the bottleneck. If you are at the limits of
connectivity it will be. (multiple walls, distance, etc.)
3. If both devices support 40MHz (which is probable), 40 MHz will offer
you better throughput - ESPECIALLY under noisy conditions. (Imagine
you are at a noisy party, if someone is talking in a low tone, you
may not hear them, while if they are speaking in a higher tone you might -
or vice-versa. More bandwidth = more tones to hear and get a signal
from). Because 40MHz gives you more maximum bandwidth it gives you
more maximum throughput. (If you were in an office environment where
you needed to cover a large area and controlled all sources of noise,
you may not want 20MHz). As an aside, if you can't get a connection
with other devices when using 40MHz, drop it to 20MHz - I have seen this as an issue, but only on some equipment.
You can measure if 20MHz or 40MHz is better for you by transferring data
to and from another computer on your network which is connected via
Wi-Fi. You will find that 40MHz is better though. Wireless transmission
is not restricted to one channel, but most devices (including 802.11n
devices which are common Wi-Fi) only use 1 or 2 channels to "play nice"
with neighbours. Also, channels "overlap", so if your neighbour is on
channel 1, and you are on channel 2, you will get a lot more interference
than if you are on channel 4 or higher (but less then if you were both
on channel 1). There are only 3 (or maybe 4) non-overlapping channels
depending on where in the world you are (slightly different frequencies
4. If you could actually get 72mbps on your 20MHz channel, that would be
correct. In the real world you can never hope to get anywhere near it,
although you should still get more than 20Mbit, in which case the ISP
will be the bottleneck. If you get 100Mbps from the ISP, then yes,
your Wi-Fi device will be a bottleneck. Unless you have an agreement
with your neighbours about bandwidth allocation and it's an issue, use