Without having to overclock the chip. I can tell you its about 10000 percent faster and more powerful then your current duo chip. Games on avg take up 60 percent of CPU soo you got breathing room, on your DUO , it goes to 100 percent and bottlenecks... You will not see a difference between a 2500k or 2600k overclocked to 5Ghz or what not. Its already fast as heck,, no difference only people with DAW or Video editing will notice the difference.
Your taking a big leap from a 2006 chip to a 2010 2011 chip. Trust me its a night and day difference, ... and make shure to couple a good video card with it, and your FPS will be 60 and never drop no matter what scene or what @ 1080p 8xAA 16xAF vsync on,, high quality... gl But if you want the ultimate upgrade, put a SSD with that 2600k Then you will see a HUGE difference ... and apps and what not popping up in 0 seconds,, My dads rig, 2600k and 128GB SSD SATA 3 , I dont feel a difference from his to mine, except the fact his stuff comes up quicker cuz you have 0.000001 latency, so files are read instantly.. Its worth it even if you pay 300 dollars for a SSD. Its the final step in making your rig NON bottlenecked...
gl