Re: Is Wireless N worth it
1) it is bits and you do not have 20MB for broadband. You are confusing it with 20Mbps. 20Mbps = 2.5MB. 54Mbps = 6.75MB. And although you are "losing speed" at the modem end this is normal. Internet (wan) speed hasn't caught up to LAN speed. Well it has but those technologies are meant for network providers, not residential customers.
2) from what I remember reading about N when it was first coming out is that it both improves speed & range.
3) the difference between a nic & a usb is that just that. You have to open up your pc to install the nic. You just plug the usb to the outside of the pc. They both perform the same function.
4) you can always buy a different cable modem as long as it is supported by the cable company. I don't know what you mean by rubbish, but if you mean better connection then it's doubtful. If you mean it doesn't have many features then yes I would probably agree.
Re: Is Wireless N worth it
the only issue i have with my cable modem is the ugliness of it, and that if they sell it to you it is overpriced.
You need to call your cable provider and find out if they will actually let you provide your own cable modem. Some will NOT due to problems with tech support. My provider actually flipped back and forth on this. First they did, then they didn't. Now they do again.
Also you are most certainly paying a fee to rent the modem they give you. If you can supply your own, first find out if they sell the ones they have and for how much. Then buy your own. I'm renting mine for $3 a month, and thinking about buying one since i've been renting it for almost two years now.
As for the g vs n. The only way you will notice any improvement is if you have a network of pcs at home. File transfers between different pcs is multi-times faster, and a usb dongle will keep up with it easily as long as you are using a 2.0 port, and you haven't got something else hogging the bandwidth on the same port. And i mean hogging it as in a usb monitor adapter.
Finally yes N improves both speed and range, but you have to have N on both ends, and usually by the same manufacturer thanks to the "draft n" specs. In theory firmware updates should have made them all compatible by now, but in practice i have a usb n dongle, and there are no firmware updates for it.