You have 2 choices, 4770k/4790k (4790k will need BIOS update). But these are 4 core with hyperthreading so give 8 threads.
Otherwise you are looking at a huge upgrade to new X99 motherboard, 5820k and DDR4 which is a true 6 core with hypertheading. You really want to be sure it's worth it, try and find some benchmarks. I don't think the cost is worth it