I have one of the 1st generation 13" Aluminum MacBooks (low-end 2GHz model) and I've got to say that I have very few problems with Flash or YouTube. In fact, I can watch the 720p content on Hulu and CBS without any issues. I'm running wireless on a Apple Airport Extreme with an extended network using an Airport Express (both set to 5GHz, 802.11n only mode).
Frankly, the only significant playback issues that I've seen were caused by network problems. I think the quality of your ISP and your network has more of an effect than anything else (unless the video servers are having problems, which certainly can happen). High resolution Flash video is processor intensive, but any of the recent MacBooks should be able to handle up to 720p as long as the browser is the only thing running and you have a good network connection.