Aaack! Difficult Poll! No Biscuit!

Motherboard determines what kind of CPU _and_ what kind of Video card you can put in it.

But: CPU choice determines what kind of motherboard you can use.

But: Upgrading a video card can force both a cpu _and_ a motherboard upgrade to use it.

My way might not be the best way, but I put together a virtual rig, then as I get money I buy physical parts to replace the virtual ones.
As to what makes the determination for the virtual rig componants, it really (for me) comes down to cpu. Cpu determines the motherboard, then the motherboard determines what video card I get.