I thought a 120g would have been big enough seeing as I only play Fallout regulary (I can delete the other games off if I need too) and I'm sticking with my old Playstation design.
The price difference between 160GB and a 640GB one is £10 . . .
Also I'd recommend going for a 7,200 RPM one for faster access, if you have the money. Or if you've got even more money then go for a 10,000 RPM one, but these are more expensive and harder to get hold of.
EDIT: If you've got even more money, an SSD is a viable option too.