In general, .avi will be a smaller file than an .mpg, but mpeg's are easier to put on a video-cd for use in a dvd player. Another thing, .avi files tend to have special codecs that you computer doesn't come with. The free codec download from
http://www.divx.com will help.
http://www.xvid.org has another codec that should help.
Codecs are like keys to help you decode the video.
As far as the wait goes, be prepared to wait quite a while. A few days is not uncommon. The estimate that WinMX gives you just shows how long it will take to get teh file if the speed you are getting it at stays constant. If another user starts you, your time to completion will decrease.
To find more users, right click on the file in your transfer screen, and choose "find sources"
I suggest you download the videolan media player from
http://www.videolan.org/vlc
It's very good at playing troublesome movies.
