Almost any internet speed package beyond the most limited economy speed will be fine for streaming TV. Normal comcast HSI packages will work fine unless you have other users who are heavily into torrents. Your N router will work for wireless streaming as long as; it is not overloaded by other users, the distance between the tv and the router is not too great or has walls between them at a bad angle, and you do not have some interfering device like cordless phone or neighbor on the same frequency (usually fixed by changing channels). The roku will provide SD to the tv and you may be happy with the result of how the tv displays it. If you are not, you would need to upgrade the roku, but try it first, you have nothing to loose.
What you are asking to do is fairly common and not terribly difficult.
If you dont have an hd device you cant get hd, you will had over your tv if you have an hd tv and you have a hd cable box. your blue ray will also be in hd, but if you don't have an hd roku your going to get hd, but try it out first you might not notice the difference. HD roku has nothing to do with the streming signal the box itself the roku if not in hd cannot send its signal to the tv in hd. You also need to check to see what speed internet you have, if your planning on streaming to you tv and roku and dvd player you might want to increase your speed, if its working fine now then you probably don't as the tv has nothing to do with the wireless signal you are getting from comcast.
alas, i don't have a hd roku player. i didn't know it was required to get a good streaming signal. currently my basic "wireless" system only supports my "dumb" tv, basic roku device and dvd player. i was hoping not to replace any device or increase my internet speed in order to upgrade my entertainment level!
your internet speed should be fine, but comcast cant guarantee that your wireless router will get to your tv or your other devices thats up to your router. YOur roku has nothing to do with comcast, do you have an hd roku, if you do then it will broadcast to your tv in 1080 if your wireless signal can hit it.
i will be purchasing a 46" samsung smart tv - 1080, 240 hz. i would like to use my current internet broadband speed, wireless n router, blu ray, and roku device. is that possible? if not, what internet speed is required? will i need a wireless adapter? will i need to upgrade my roku device to support 1080?