Lets see where to start. I have/had charter. I just upgraded to the new 20mbps internet. So now I need a new modem and router or a combo. I am not really sure of the best setup or if what I have so far is really any good so here is what I have so far:
As you are getting an up to 20mbps feed you might as well go for the latest N wireless rather than G -- the only reason not to is that you must be sure your wireless adapter is also N and from the same brand.
I don't know much about cable modems. I'm surprised your ISP isn't offering their own -- might be a good move as they can't then blame problems entirely on your hardware.