I started working on what I consider to be a 100% run in Fallout 3 a few months ago.  This included doing/finding:
 
-Every quest
-Every side quest
-Every DLC
-Exploring every building
-And of course, getting every trophy.
 
I've now spent 179 hours on Fallout 3 and am nearing my completion.  It's such an amazing game but also one of the most frustrating experiences of my gaming life.  From what I've read, the game tries to remember everything that you've done or found so once you get far into the game, it starts running like... well, crap.  The game runs fine when I'm in indoor environments for the most part.  If I go outside onto the wasteland or into the swamps in Point Lookout, it starts to freeze at least once every twenty minutes.  I love the game but I can not wait to be done..  I only have 10 locations left in Point Lookout, the last two missions of the vanilla game, and all the Broken Steel content.  The thing that makes me want New Vegas the least is the performance of Fallout 3 itself.