You would probably need at least 3Mbs download speeds. You can do a search on Google (Download Speed test) to determine your speed.
I just tried www.speedtest.net and it indicated a download speed of 3.5Mbs. Hmm, I am paying for 7Mbs. There are many variables involved and I am in a fairly remote area so I am not going to complain.
When comparing DSL and cable keep in mind that with DSL you have a fixed bandwith and with cable you are sharing bandiwidth. Therefore if you live in a neighborhood where many people are on cable, you may see variations in your download speeds versus DSL would remain constant.
Yes, ROKU has different models. I'm not savvy on all the variations, however I do know they have a wireless unit and one that can also be attached to a wired ethernet network as well. I chose the less expensive models and they have worked flawlessly on my wireless network. I have two of them, one for the living room and one for the bedroom. I think they were around $60 each.
Shop around. This is the wave of the future. TV's, Blueray players, gameboxes, etc are pretty much all internet TV ready.