The multi core thing isn't true - I have a quad core and Fallout 3 runs fine on it.
The game is actually optimised for multi cores, as it says on the box.
Depends on the system.
On desktop rig with a Core 2 Quad Q9450 I had no issues whatsoever except for the occasional crash.
My laptop has a Core i5-2410m (dual core + Hyper Threading) the game would constantly freeze until I edited the ini file.
Don't know about a quad core i5-2300, i5-2400, i5-2500, i5-2500k, or i5-2550k CPU since I don't own one. None of those CPUs has Hyper Threading.