Now that christmas morning is dead and finished (new mic, yay!), I have a question that needs answering.

I only have enough money for one game (and some left over to Pre-order Mass Effect 2), so my question is, what should I buy?

I own an Xbox 360.

Favorite genres include:
RPGs
Shooters

I've been looking into buying Fallout 3 now that the GOTY edition is out, but I'm not too sure.

ANY SUGGESTIONS?