if you have to OC A cpu to 4ghz to play games then something tells you got a good gpu but you bought it for your ego not to power your screen resolution. The higher the resolution the more the work will be on the gpu. Your cpu might bottleneck at a low resolution but not at a high resolution. Most bottlenecks occur not because the cpu is too slow but because the gpu can run away from it due to the light work on a low resolution. Any core2 cpu or Amd x2 cpus and up with a clock of 3Ghz will happily run most games. Paying 130usd for another core2 would make no sense. You might as well get a Amd x4 cpu of a tri core. But a upgrade from a dual core to another dual core is a waste
I must admit I had a hard time digesting some of the things you wrote there...
Let me tell you a bit about why I overclock the cpu that far.
I've went through lots of upgrading. The first pair that I have is an E2160 OC'ed to 3GHz and 3870, it was good, and balanced, but that time I was using a 1440x900 19" monitor. Then I bought a new 1920x1080 23 inchers. The gpu struggled to produce a playable fps playing COD4 at the native resolution.
I upgraded to a 5850, and it's much better at running COD4, but then I install Battlefield: Bad Company 2 and the cpu can't keep up. Stutters and frame skips happened here and there. The fact that my GPU doesn't support hardware physx, only put an even harder load to my cpu. It's at 3GHz, by the way.
I found out that the cache and processor architecture that of a Dual-core Pentium is different to a Core 2 Duo, so I get the cheapest Wolfdale I can get. The E6500 at 2.93GHz. It turns out that the doubled L2 cache provided enough fps juice, but still not as smooth as 60fps, but I can't dish out more money to get a quadcore at the moment, so I stayed with the duals, but I overclock it to the maximum point it can take. And, the fps is smooth now. So I've went through some horrible bottleneck and it's all to deliver playable fps to my monitor. I don't see anything wrong with that.
As for the upgrade recommendation, aambrozai said that he's on a budget. So I believe any quad-core that has a minimum price of $163 (http://ark.intel.com/ProductCollection.aspx?familyId=28398) is not an option. A bigger cache dual-core would help him on achieving higher fps in Crysis 2, and balance the already powerful gpu with the cpu. In my opinion, it's more less of a sense to buy an entirely new system based on an AMD X4 or X3, and it wouldn't help with the budget situation.
I aggree that any Core 2 @ 3GHz can run games, but at what detail level? Remember that E2140 is not a Core 2 Duo, and has a mere 1MB L2 cache. Provided that you have a powerful gpu, it's best to avoid any bottleneck by clocking the cpu as fast as you can so the gpu can maximize the pixel output without waiting.