If I played GTA IV/Fallout 3 on my PC with my Xbox Live account, can I buy GTA IV/Fallout 3 for the Xbox 360 and play on the same account, will I have my save data? (No mods)
No. GFWL offers very little in functionality between Windows and Xbox. If you want to transfer your saves, you can try to use a USB drive and a save conversion program, but this is unreliable. I previously tried to transfer a Skyrim save between Xbox and PC with limited results. Your mileage may vary.