There is a difference between (container)format and codec. Xvid-encoded stuff in the .avi container won't work (technically it can, but it is a very bad idea.) Lagarith encoded footage in .avi on the other hand will work.
What you are looking for isn't a container so much as a codec. For editing purposes, you want to be editing with losslessly encoded footage (done with a lossless codec, such as lagarith or huffyuv.) There are other editable codecs that aren't lossless, but lossless is just that...you don't lose any quality.
Lagarith and Vegas are best friends
Be aware that lossless=big, you don't want to encode entire episodes losslessly unless you have a lot of free space (and time) on your hands (a single lossless dvd ep at 720x480 is about 2-4 GB)
lastly, Read ErMaC & AbsoluteDestiny's Friendly AMV Guides Lovingly Overhauled Largely by Zarxrax
. The amvapp should have everything you need and the guide goes over how to do footage prep, project/render settings and encoding for distribution