I have been working with setting different friends, family, and a couple of nonprofit orgs with broadband over the past year…
Unless you are a heavy downloader, you will never notice the difference between 12 and 20.
As has been said by others, typical home cablemodems produce somewhere around 7, and this is quite sufficient for video Skype, watching Netflix, and other seemingly bandwidth-intensive things.
I recently went to 25/25 service (25mbs downl/25mbs up) and love it, but in my case I have teenagers who are hogging bandwidth, my wife is always watching Netflix, and I still am able to do what I want at high speed. When considering your speed, remember that most services provide asymmetric bandwidth; the download speed is sometimes several times faster than the upload speed. This shows up especially when you do video chat.
On the lower end of the spectrum, My sister-in-law uses DSL and has around 1mbs and has been getting along well with that, though her network has a distinct lack of zip.
I have an aunt who just signed up with Clear and they gave her a cool home modem that uses 4G cell phone service (if available) to give her around 6mbs down and 700kbs up.
Her service is advertised as $35/mo; there is nothing to install in the house. The device is both a modem and a wireless router; no wires at all.
The down side is that video Skype is grotty for the person she is talking to (i.e. me), because her upload speed is so low.
Stick with 12mbs if you are happy with it. If you are a light bandwidth user and want to go totally cheap, Clear seems good if it is in your area. DSL can be cheaper, but it is slow enough for you to really notice it.