I've tried doing this a few time's myself. I have a problem maintaining the same quality. Here's one way that I know of. I'm sure there's plenty other ways out there that I don't know of.
If you have Firefox, download an add on called "download helper" this will run in your main tool bar. Use this to download the video playing. It will be an FLV file. Then use a program something like "Riva FLV Encoder" to conver that FLV to an mpeg.
Good luck!
If I find a video i like, I'll usually just save it and replay with FLV player.
sorry if this is basic stuff you already know
later dudes