Dual core CPUs were a desktop novelty in the first half of 2005. Now, with the introduction of the Mac Pro (see one unboxed), dual core is officially pass. Quad core-- at least in the form of two dual-core CPUs-- is where it's at for desktop systems.
And sometime early next year, the first true quad core CPUs will hit the market.
I think there are clear multitasking benefits in a dual-core configuration for typical computer users. All you need to do is run two applications at once, and who doesn't do that these days?
However, the benefits from moving to quad-core and beyond are less clear. Effectively utilizing 4 or 8 CPU cores requires extremely aggressive multithreading support within applications. How aggressive? Rewrite your entire application in a new language aggressive. That's a much more difficult problem. It's also not a common optimization, except within very specific application niches.
Dual CPU desktop systems weren't twice as fast as single CPU desktop systems. But they were a substantial, worthwhile speed bump. With quad CPU systems, we've hit the point of diminishing returns.
Current benchmark data definitely bears this out. I distilled results from these GamePC and TechReport reviews of the Opteron 275 (dual core 2.2 GHz), which also included the Opteron 247 (single core 2.2 GHz). It's an apples-to-apples comparison between Dual and Quad configurations of an Athlon 64 running at the same speed-- 2.2 GHz.
| Dual CPU | Quad CPU | ||
| 3D Studio Max 7.0 Radiosity Render | 239 | 144 | 1.7 x |
| POV-Ray chess2.pov | 144 | 87 | 1.6 x |
| Cinebench 2003 Rendering | 571 | 1021 | 1.8 x |
| Alias Maya 6.0 Zoo Render | 49 | 43 | 1.1 x |
| Photoshop CS Filter Benchmark | 146 | 131 | 1.1 x |
| Flash MX 2004 MPEG import | 37 | 35 | 1.1 x |
| Windows Media Encoder 9.0 MPEG to WMV | 125 | 119 | 1.1 x |
| Xmpeg/DivX encoding | 71 | 75 | 1.1 x |
| LAME 3.97 WAV to MP3 | 69 | 67 | none |
| Apache 2.0 10k user stress test | 1397 | 1478 | 1.1 x |
| Apache 2.0 50k user stress test | 1346 | 1875 | 1.4 x |
| Sysmark 2004 | 226 | 242 | 1.1 x |
| Half-Life 2: Airboat chase | 95 | 96 | none |
| Doom 3: Site 3 timedemo | 164 | 166 | none |
| 3DMark05 | 5244 | 5244 | none |
I eliminated most of the synthetic benchmarks; I tried to focus on real desktop applications that people actually use. The Sysmark 2004 results are particularly telling.
However, the results I did find are so poor that I wonder if any quad CPU system is good for much more than bragging rights. Of the desktop apps, only three truly benefit from a quad CPU configuration: 3D Studio Max, POV-Ray, and Cinebench 2003. Notice a pattern? Rendering and encoding tend to parallelize well.
Unless you're often running a specific application that is optimized for multithreading, there's no compelling reason to run out and buy a quad-CPU desktop system today. And I don't see that advice changing over the next few years. At least, not until the state of software development changes quite radically to embrace multithreading across the board.