So let's tackle theoretical CPU performance first. A new GPU will do nothing to address that - in fact, it could well make it worse as the gap between being GPU-limited and CPU-bound grows wider. Games are becoming more dependent on a certain level of CPU power, and if your processor falls short, the gameplay experience is marred with intrusive stutter. It's actually fairly easy to hit CPU limits with a Sandy Bridge quad-core processor - born out by this video where Ryse, Crysis 3, Assassin's Creed Unity, The Witcher 3 and Far Cry 4 all exhibit performance issues - and where frame-rates actually lose out in some cases to a modern dual-core i3.
But we've noticed that the times are changing. And that's actually a lot more difficult than it sounds: the graphics card dominates conventional gameplay benchmarking, providing a ceiling to performance that makes discerning CPU deficiencies quite challenging.
Our testing is a combination of the theoretical alongside practical reality - our most comprehensive effort yet to discern just how important the CPU is to your PC gaming set-up. We've concentrated on the 2500K here, but almost all of the observations we're about to impart could apply to its well-regarded successor - the Core i5 3570K. Our question: is now the time to upgrade. Five years on, many PC gamers refuse to relinquish the classic i5. It was the right processor at the right time - providing immense levels of single-core performance (good for old games) along with a quad-core configuration that serviced more modern titles admirably. There aren't many pieces of PC hardware that actually qualify as genuine gaming icons, but Intel's vintage 2011 Core i5 2500K is surely one of them.