From hothardware:

A few weeks back, we discussed whether a new GPU like the GeForce GTX 660 could breathe new life into an older quad-core gaming system built in mid 2008. The answer concluded was definitely yes; but many readers asked to reconsider the question, this time using a lower-end dual-core Core 2 Duo. The Core 2 Duo CPU chip used was a first-generation C2D part based on Intel’s 65nm Conroe core. It’s clocked at 3GHz with 4MB of L2 cache and has a 1333MHz FSB. The CPU was paired with 3GB of DDR2-1066 memory. The long and short of it is, you can upgrade the graphics card on a six year-old dual core machine and expect to see a noticeable improvement in game performance — significant gains in fact, up to 50 percent or more.

We’ve done so. And just for fun, we’ve added overclocked CPU results into the mix as well.

The Q6600 we originally tested was one of the best-selling enthusiast processors of its day, but dual-core chips were very much the norm. At the time, there were only a handful of games that could even take advantage of four cores and both Intel and AMD priced by core count. That’s changed now, with multiple games using up to four cores.

So, can older Core 2 Duo’s still keep up? Let’s find out. The only Core 2 Duo CPU I still had on hand is an Intel E6850. It’s a first-generation C2D part based on Intel’s 65nm Conroe core. It’s clocked at 3GHz with 4MB of L2 cache and has a 1333MHz FSB. If you were to pull Intel’s full suite of Core 2 Duo consumer SKUs, the E6850 would have ranked at the upper end of the heap. Performance should be somewhere between the 45nm E7600 and E8400.

We tested the E6850 and Q6600 in the same Asus X48 Rampage Formula motherboard with 3GB of DDR2-1066. An EVGA GTX 260 and GTX 660 were used to compare graphics performance. Our overclocked figures were measured using the Q6600 (SLACR core) with its FSB bumped to 1333MHz, up from 1066MHz.

As before, we present performance figures for the GTX 260 and GTX 660. Overclocked performance with the Q6600 at 3GHz and the GTX 660 is also given. The primary question we want to answer is how much a new GPU can help the performance of an older dual-core CPU. In our default configuration, the Q6600 has twice the cores, but the E6850 enjoys a 25% clock speed advantage.

The Q6600 is, on average, 21% faster than the E6850 when both chips are running at stock clocks. Overclocking the Q6600 also yields positive results. Games that are completely GPU-bound, like Civilization V, gained no performance, but frame rates in Battlefield 3, Borderlands 2, and Shogun 2 increased a further 16%.

Here’s the short, non-math version: A quad-core is significantly better than a dual-core for modern games. An overclocked Core 2 Duo quad-core is meaningfully faster than a stock-clocked variant.

The Cost of Upgrading

Intel Core 2 Q6600 chips aren’t available new these days, but Ebay has a ton of them, regularly priced between $50-$70. If you plan to overclock a Q6600, you’ll want the G0 stepping (SLACR). Other quad-core variants are also available, including chips based on the 45nm Yorkfield CPU. Price and availability are obviously highly variable and the usual caveats apply when buying from Ebay.

Is a new CPU worth the price? I’d say yes — especially if you’ve currently got a dual-core CPU in the 2.2 – 2.6GHz range. The combined cost of a used Q6600 and a GeForce GTX 660 should still come in below $300 while delivering far better performance than any bottom-end desktop you might assemble for that price tag.