What format/player is best for videos

[MarcLodge]

Hi all,
Newcomer here with first post. I have just bought an xda mini running wm5 (I think) into which I have sucessfully installed a 1Gb mini S card.

My daughter has an old XDA II running windows, but I've no idea what version or where to go to find out - sorry. She too has a 1Gb SD card in the top.

What I would like to do is to be able to play video files on both PDAs. I have some AVI files on my laptop which I have converted to WMV files using a converter which has made them reasonably smaller and these play fine in Windows Media Player, but I have a feeling that although this works, this is not the best way to do it.

So.....

What file format is the most efficient for PDAs? (Mpeg, DIVx, xVid, AVI, WMV)
What video player is the one most commonly used?
What converter is best to use to convert AVI (or WMV?) files to be able to be played on the PDA?

Apologies for all the questions. Hope you can help.

Marc