your point about sharing with others in your neighbourhood is really a moot poit, since all bandwidth is shared at some point, its the whole basis of the internet. I'm not sure how much upgrading to the cable networks the cable companies in the US have done, but the ones here in Canada have done such a great job with upgrading, though they have always been more up to date than their American counterparts, but not by much anymore, anyways i have a 5Mbit cable connection and its simply awesome, even shared among 3 wired and as many as 3 wireless connections, the bandwidth i get is shared between prtty much the 30 or 40 houses on my street and thats it, most of which are on cable, though most are only on 3Mbit connections. My grandparents have DSL at 3Mbits and the speed is more consistant than on cable but the average speed is much slower than mine thay only have one computer.
My cable modem is on ethernet connected to my 3-4 year old router (better than the current version of the same router), DO NOT USE USB, uses too much processor resources, Ethernet uses minimal processor resources and can be connected to a router.
If you can get 4Mbit or faster in your area on cable, even if you have to buy the modem (we had to buy the 5Mbit modem for $80CDN and the 5Mbit service was the same price as the 3Mbit service, but needed to buy the modem, needed to meet a new standard or something) buy it!!
Good luck.