Make any movie format play through iTunes, this doesn’t alter the original movie format at all.
It doesn’t make movie playable on your iPod though, that takes a different application.
What do you mean Quicktime won’t play all your media files? You need to pick up the excellent Perian from here. It will allow you to use:
– AVI and FLV
– 3ivX, DivX, Flash Screen Video, MS-MPEG4, Sorenson H.263, Truemotion VP6, and Xvid
– AVI support for: AAC, AC3 Audio, H.264, MPEG4, and VBR MP3