Basically the ROKU is limited by your wireless settings/provider. I learned some real good lessons about the difference in provider content vs. how much you spend on a TV, lol (I HATE CABLE).
Depending on the speed, and the possibility that watching movies could push you over a network usage cap, I would proceed with caution. It's easy to forget the SIZE that a HD quality movie can be. (not even sure if that is an issue for you?)
I have a 42" LCD HD 1080p, and anything under 3 dots is unwatchable. Even 3 dots will make u wonder if you made the right decision. I learned how to force load content at the highest quality (works for some, not for all) but at 1.5 Mbps loading times (and re buffering) can be VERY irritating.
That said, I LOVE my ROKU. Streaming watch-it-now wirelessly to my TV is one of the coolest things that I have done for my house and budget. Imagine 12000+ movies for $100??? The network set up was easy.
If you can say without a doubt that you can get 3 Mbps I would recommend trying it. With a SD TV I would even say that quicker.
Remember, you will also be sharing with your computer. So if you are a gamer or P2P user and your laptop is always on--you have to calculate the "available" bandwith. Speed tests on the internet can be a good indication, but it depends on where you go. Try to get an average rating between several (don't forget to check out your own providers numbers).
Very funny Scotty now beam down my pants!!!!