Seriously: the GeForce GT 520M, launched January 2011, wants a full PCIe 2.0 x16 interface. Same with the Raedeon HD 6330M. You could probably get away with just 8 lanes if you had to, but not only one.
Connecting a GPU with just one PCIe lane isn't the biggest problem. You'll just slow down data exchange between the CPU and GPU (mostly loading textures and vertex positions).
If your game mostly relies on shaders and renders lots of rather static stuff, you'll mostly just get longer loading times but FPS shouldn't suffer too much.