12fps for the E8400, 22fps for the i3-3220 and 40fps for the i5-2500k. Performance in Crysis scales almost exactly with available processing power all the way to the i7-3960X.
Most games are heavily dependent on single-core performance, downgrading from 3GHz on a newer architecture to 2.4GHz on an older architecture won't do you much good.
On the Intel side, anything from Sandy Bridge i3 and up should handle most games that are still playable on a GTX275 reasonably well. If you want to play some of the newest or near-future games though, you may want to aim for an i5.
However I'm not that sure your CPU is bottlenecking now. One of my setups has a Pentium G860 tied to a HD7850 and there are in fact not much games which have CPU usage at 100 percent. So the GTX275 might be the weakest part, besides from the fact that your CPU isn't the best either.
However I'm not that sure your CPU is bottlenecking now. One of my setups has a Pentium G860 tied to a HD7850 and there are in fact not much games which have CPU usage at 100 percent.
If you run a game that's heavily dependent on single-threaded performance and runs the bulk of its code in a single thread, you can run into CPU bottlenecks on a quad-core/thread CPU even while Task Manager is reporting only 25% CPU load. With most games running a "busy wait" in their main thread, one core is always stuck at 100% usage on whatever core it is running regardless of what is happening, which makes it that much more difficult to determine whether the thread is bottlenecking on the CPU or something else.
The simplest bottleneck test is to either lower or increase game resolution with vsync off and see if that makes any difference. If the CPU is the main bottleneck, performance will remain almost exactly the same. If the GPU is the main bottleneck, performance will be strongly correlated to pixel count on the screen.
12fps for the E8400, 22fps for the i3-3220 and 40fps for the i5-2500k. Performance in Crysis scales almost exactly with available processing power all the way to the i7-3960X.