so many formats, what's the best? and when do i need a video server?

[sely]

There are about 5 videos i need to put on my site. the source of the video is asking me what format i want them in. the information i've found goes as follows:

AVI - very common, windows only (for the most part), most browsers
Windows Media (wmv) - Windows only unless you download a plug in.
MPEG - Most popular, cross platform
QuickTime - Need plugin for windows
RealVideo - streaming video
Flash (shockwave) - need plugin but preinstalled with most recent netscape and i.e.

My questions are:
1. If MPEG does it all without a plugin, why bother with any of the others?
2. Under what circumstances would i need to look into a video server as opposed to just posting the file?
3. Last i heard AVI was rather archaic, is this true?

thank you for your help.

[gooogle]

it must be flv, but you don't list it here.