Well I'm not sure about the new DLC for Fable 2 as it hasn't been released yet. As for the first DLC pack (Knothole island) I purchased it before I had finished the main game. The good thing about that is that I was able to access new items/weapons, etc. that are only available from the DLC. I used those to help me in the main quest. It's not necessary, but definitely helpful.
I'd wait to see if you actually enjoy playing the game though before deciding to get the DLC pack. I was really having a great time playing Fable 2 and that made me decide to get the DLC pack as early as I did. I'm still playing all the sub-quests even after completing the main quest and DLC quests as well (looking forward to the new DLC as well!)
I don't have Fallout 3 so I can't comment on that.
starcraft: "I and every PS3 fanboy alive are waiting for Versus more than FFXIII.
Me since the games were revealed, the fanboys since E3." 
Skeeuk: "playstation 3 is the ultimate in gaming acceleration"
