If you prefer to download in *.flv, the size and the quality is less than *.mp4. however the download can be faster.
below is step by step downloading youtube video using keepvid.com in flv or mp4 format from youtube
1. klik keepvid.com
2. in youtube, be sure you copy the video's link
3. paste in keepvid.com input form
4. now choose to download in flv or mp4 filetype
5. wait the download process and done.
Figure-Figure step by step above :
Figure step 2
Figure step 3
Figure Step 4
beside keepvid.com, this link also can be use, ---->techcrunch (but by the time i post this posting (2009/03/18), it was disable and say's it will be running soon)