From YouTube's online help:

What's the best format to upload for high quality?
We recommend the MPEG4 (Divx, Xvid) format at 320x240 resolution with MP3 audio, at 30 frames per second. Resizing your video to these specifications before uploading will help your clips look better on YouTube.

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Is there a limit to how much I can upload?
Currently, the only limit is 100MB and 10 minutes in length per video (there is no length limit for Director accounts). We don't have any limit for the number of videos you can upload, and don't foresee adding a ceiling anytime in the future. So get crankin' and upload as many videos as you'd like-it's more content for the whole YouTube community to enjoy!