When the Core2 CPU's first came out, PC Pro (British PC magazine) bunchmarked about every CPU they could find. A 2.4 GHz E6600 was about 65% faster than an EE P4 running at 3.8 Ghz. So assuming a Wolfdale is 15% faster than a Conroe and an SB is 15% faster than a Wolfdale, that is 3.8 GHz * 1.65 * 1.15 *1.15 = 8.3 GHz at the least.
I used to have a P4 530 3GHz desktop. Then I bought a laptop: Core 2 Duo 1.66 GHz (Still have it). The laptop turned out to be faster than my desktop even on a few single threaded benchmarks (such as POV-RAY). I estimated the core 2 duo performance to be at around 3.4 GHz P4 on per thread basis. I did not compare them on wide array of benchmarks, but at least on some of benchmarks core 2 was more than twice as fast as P4. (EDIT: more than twice on equal frequencies)
While my math won't be as nice as your JSC I think it should be a bit higher then that. As pointed out, the C2D was roughly 2x as fast as the P4s. If the 2500 is 3.3/3.7GHz (lets say 3.5GHz for easy math.) then you have to be looking at 7GHz at least. I'm pretty sure Intel has made some improvements to the C2D arch between now and then so while 8GHz+ is close I think its probably closer to 9 or 10GHz.
Careful e56imfg. From what your typing it sounds like you think its impossible for a less efficient arch to keep up with a more efficient arch no matter the clock speeds involved. This is obviously wrong as Intel stayed close with the P4 vs the Athlon x2. It's all about Instructions Per Clock. (IPC) If you have a low IPC CPU, you can keep up with better IPC chips just by having more Cs. (higher clocks.) It is possible to have a P4 965 keep up with a 2500K assuming you have one that can reach the clocks needed.
Finally, even though I said it needs to be higher then 8+GHz, keep in mind the OP is asking about gaming only. For the most part you only need fast enough to drive the GPU at the required res. Seeing as the OP didn't say the res we don't know for sure. Another thing is that the 965 doesn't need to keep up with the 2500k, just provide similar frame rates. Seeing as we are talking about gaming and not actually similar CPU speed my gut says even a 6GHZ+ will do just as well as a 2500k when gaming at 1080. There are still plenty of people gaming with E8400s which are around 3GHz.