If you've got live (Which you must, if you're getting MW2), Gamestop is a good deal for MW2. You can get the game for $50, and it comes with a free 1600 point card to download one of the map packs for the game (or possibly some of the arcade games going on sale next week...).
For Fallout 3, I'm not sure. Keep your eyes peeled at Gamestop's coupon codes; I got the GOTY edition for $32 a few months ago on their site with my Edge card and a coupon code.