Yes. You can definitely post your muvees onto Youtube!
I find it best to save my muvees at 640x480 in WMV format. I also try to make sure that my muvees are less than 10mins in duration, so that I won't run into problems with file size and have to re-convert them.
If you're using autoProducer6: Select the 'Save to Computer' tab, and then click on the radio button that says WMV.
If you're using muveeReveal: Select 'Computer' and then click on the first tab that says WMV.
I hope that this helps! Enjoy your muvee making! Feel free to write in to us at support@muvee if you run into more problems!