Boot Animation Database

Here's a few threads on the subject:
[Boot Animation] [12/04/11] A Huge Collection of Boots ~ Updated - xda-developers
[BOOT ANIMATION] The Droids - xda-developers
Boot Animation Index

Most will work as is, some will require you change the png image sizes in the part folders to 320X480, though that usually isn't necessary. ;)

Yeah the reason to do this is if the animation appears larger than your screen size. But most of the time android will edit it for you.