You have your work cut out for you. I think about 6 months ago it was approaching 400. I imagine now you would be looking at close to 450 in the US alone. Add another couple of dozen for Japanese and European only releases.
Looks like we are well over 700 now even after taking out some of the duplicate releases (Limited Editions and the like). I just need to update the list with this week's releases.
I'm somewhat surprised that people only think there are around 400 games for the Xbox. It's going to have the third largest library ever for a console. There's a topic on this as well.
Oh, and I think there are at least another 200 Xbox games currently in development for 2006 and 2007. I know Major Nelson said that there were more games coming out for the Xbox in 2006 than in any previous year.