After writing that article, I decided to troubleshoot the mail-checking slowdown problem one more time, and I realized that I hadn't applied some basic tweaks to this installation of Windows XP Pro. Once I set the OS scheduler preferences to optimize for "background tasks" instead of "applications," my mail problem was largely resolved. I also used registry tweaks to increase the size of the system disk cache and to disable paging of the Windows executive, and all told, my system is much more responsive now.
Now, I still think dual-core CPUs will be a great thing for multitasking, but this raises the question: How much creamy smoothness can you squeeze out of a box with only one CPU, with or without Hyper-Threading? And what proportion of PC slowdowns and performance "hiccups" are really caused by inadequate CPU power as opposed to lousy OS scheduling, hard drive bottlenecks, running out of RAM, lousy drivers, or the like? Is multitasking nirvana really just a second CPU core away? What, in your experience, has the most impact on your PC's responsiveness, and what upgrades have helped the most?