Similar to their Pentium Extreme Edition 840 big brother, the Pentium D 840 and 830 feature two CPU cores within a single casing, each with 1MB of L2 cache and the ability to run 64-bit code. The key difference is that the Pentium D dual-core processors lack the Extreme Edition's Hyper-Threading capabilities, which allow Windows XP to recognize it as four processors instead of two. The 840 runs at a clock speed of 3.2GHz, while the 830 runs at 3.0GHz. Both are more than capable of giving a substantial boost to users who juggle one or more CPU-hungry applications such as video, audio, or 3D rendering at the same time.
AMD Athlon X2 4400+ and Athlon X2 4200+: AMD's responses to the Pentium D 840 and 830 are the Athlon X2 4400+ and 4200+ dual-core processors. Both run at 2.2GHz. In the 4400+, each CPU core has 1MB of L2 cache, while in the 4200+ each CPU core has 512KB of L2 cache. Higher amounts of L2 cache result in a significant performance increase, because programs can access data stored here faster than in standard memory. Not surprisingly, it adds to the overall cost of a CPU. Aside from this, the two CPUs are identical. Each permits 64-bit operations.
For more information on dual-core CPUs and the relative differences between Intel's and AMD's approaches, check Desktop Pipeline's Seeing Double: A Primer On Dual-Core CPUs.
What isn't a wash, however, is price. The Athlon 64 4400+ costs around $650, while the 4200+ costs around $550.
While we recommend dual-core CPUs for most office environments, a number of single-core high-end processors are worth looking at for desktop PCs that see considerable use of a single, CPU-intensive application, such as gaming or audio/video encoding.
Intel Pentium 4 660, Pentium 4 571, and Pentium 4 570J: With 2MB of L2 cache and a 3.6GHz processor, Intel's Pentium 4 660 is similar in nature to the Pentium 4 670 detailed in the performance category. The slower clock speed makes it a slightly more affordable $700. That's still not cheap, but it's $300 less than the 670, which might make this a great processor for the few users in your office with extravagant CPU demands.
Similar to the 660 is the Pentium 4 571, which runs at a slightly faster 3.8GHz but features only 1MB of L2 cache. Because the 571 costs only $50 less than the 660, we're going to recommend the CPU with the larger cache. The resulting performance difference is well worth the $50. Intel also offers the Pentium 4 570J for around $650; it's exactly the same as the 571, but without 64-bit extensions. (Why you'd want to pay the same price for a processor with less functionality is beyond us.)