Gaming rearly benefits from quad cores (mostly games dont use any more than 2 cores, so 2 cores+high GHZ beats quads normally).
That beeing said, Your PC might not benefit much from a single upgrade in this case.
What im trying to say is, you will see an improvement, but it will be very small, while waiting and buying a new system, might get you a huge jump.
Id wait for bulldozer, since it "might" drop the prices on the sandy bridge -k versions.
However if you do want to get an upgrade regardless, I would get a intel 1155 cheap mobo (make sure its a P/Z chipset, and thats about it), get a 2500-k and 4 gb of cheapest ddr3 1333MHZ ram you can find. f 2 expensive, downgrade the 2500-k to something cheaper.
But really i dont advice any of that right now.