Motherboard and CPU. Theyre underpowered compared to the graphics card, you'll be CPU limited in a few games. Although a graphics upgrade will also help, id suggest you wait for that and upgrade it later.
You could get 2k for your old cpu and mobo. So with 6k you'd get a G31 mobo and a E5200. A E 7300/7400 for a bit more is highly recommended though. Or look for a used E6600 or the likes for 3.5k to 4k