Hello.
There are a lot of different codecs & whatnot, but use BlackBerry Video Converter from Seabyrd Technologies, it does what it says on the tin.
It converts every video I have tried, and will even do .vob files too.
Here's the link:
Seabyrd Technologies - Blackberry Video Converter
It has two default settings-Pearl and 8800. It work just fine on my 8300. You can adjust the screen width/height so probably work for 8900/Bold as well.
Best of all... It's free! (or donate if you find it useful-it is)
Hope this is of use!