For testing we used an Abit IP35 Pro, which is based in Intel's P35 chipset, 2GB of Kingston KHX9600 DDR memory, a Hitachi 7K1000 hard drive and an MSI NX8800GT graphics card running on 32-bit windows Vista Ultimate edition. For comparison we used a Core 2 Duo E6750 Conroe which runs at 2.67GHz on the same 1,333MHz front side bus as the E8500. You'll have no trouble finding the E6750 on sale for £120.
We also used a Core 2 Quad Q6600 which has a lower clock speed of 2.40GHz and runs on a front side bus of 1,066MHz but it packs in four cores and still manages to cost less than the E8500 at £150.
Throughout the testing we made little effort to push the memory speed as it doesn't have much impact on performance once you get up past 800MHz but it can affect system stability. We weren't going for extreme performance but instead wanted to achieve results that you should find easy to replicate.
The initial run with the E6750 was made at the standard speed of 2.67GHz, followed by overclocking the front side bus to 390MHz/1,560MHz to achieve the same 3.16GHz as the Core 2 E8500. As you would expect this overclocked E6750 had very similar performance to the standard E8500 with an extra 10W of power draw when the CPUs were under load. As it happens 3.16GHz was the limit for the E6750 on the stock 1.35V core voltage but a small amount of extra power took the CPU speed to 3.52GHz and an extra dose of voltage got us to 3.68GHz. By this stage the system power draw was 170W which isn't a huge leap from the original 135W so we have no doubt that you could squeeze more speed from the E6750 if you use suitable levels of cooling and power.
The Core 2 Duo E8500 delivered good performance at stock speed and slightly out-performed the E6750 when they were both overclocked to 3.51GHz. At that stage the E8500 was drawing 25W less than the E6750 as it was running on standard core voltage yet it was able to complete POV-Ray slightly faster than the E6750. As a final flourish we fed the E8500 some extra power and got the speed up to 4.05GHz. That was a core voltage of 1.395V which is a modest increase however we see reports from CeBIT of a Core 2 Duo E8500 running a core voltage of 1.824V with huge cooling to achieve a clock speed beyond 5GHz.
For a final comparison we ran the quad core Q6600 and found, just as we expected, that the quad cores don't deliver any benefit in PCMark05 but they perform better than a fast dual core CPU in POV-Ray. This processor also managed to overclock by a full 1GHz with a modest amount of extra voltage and it demonstrated that fans of video coding and other CPU intensive tasks should choose a slower, cheaper quad core Q6600.
If you're in the market for a dual core processor then we'd strongly recommend a new Wolfdale based E8x00 series chip over the older Conroe based E6x00 series. Both deliver exceptional performance with plenty of headroom for overclocking but the Wolfdale chips will get you more speed for less power if you run them normally and if you're overclocking, they'll go further than ever before.