Your motherboard doesn't support Second-Gen Core-i CPUs, so you can't just add a 2600K to it. Instead, you would have to replace the motherboard, too.
The graphics card is definitely the weakest link in your hardware chain. Even a $100-120 5770/6770 or GTX 550Ti would be a significant upgrade. A 6950 might be a bit of a stretch for an i3-530, despite your overclock. Still, it would offer a very drastic performance increase over the 8800GTS.
I think I'd find myself more inclined to pair a 6870 or GTX560 with your processor.