Seriously dependent on current PC config and requirements "Most cost effective upgrade for what?" Say if its for "Getting more space for storage" than its obvious adding only a HDD can do it.
If you have an i7 4770k with two 512 ssds in raid 0 with two 2 TB drives as data drives and a 8800 GTS and you plan on using your computer for gaming, I would say the GFX card.