Computer speed depends on many things. The memory, CPU and how fast in RPMs does your hard-drive access the data and then is processed by the CPU. A slow HD at 5200 RPM will slow performance when compared to a HD at 7200 RPM. A slower HD will cause your CPU to be idle as it waits for data to process. A single core CPU can only process one command at a time, a duo CPU, two commands simultaneously, and a quad CPU, four commands. Also, the CPU speed is not a good indication of performance. For example, my P4 3.4 Ghz CPU is fast, but is slower than a Duo or Quad CPU at 2.6 Ghz or even 2.0 Ghz. Why? because it can process data twice or four times more info simultaneously than a single CPU, like a P4. Each CPU processes a single line of data, a CPU Quad has four CPU's built in.
We often do a few things with our computers at a time (download a file, email, surf the net, upload, watch video etc.) so even a desktop user will see some benefit from a multi-core CPU because it can do more than one thing at a time.
For example, being able to execute four programs simultaneously at 2.4 Ghz is WAY faster than four programs one after another (not simultaneously) at 3.2 Ghz. You're likely to find that the quad-core CPU gets more done in the same amount of time - so it's "faster", even though it has a lower clock rate.
A Q6600 (quad core 2.66 GHz) will outperform a P4 3.0 GHz by so much it isn't even funny. The best performing reasonable CPU available is the E8500 (3.166 GHz). The fastest processor in the world is made by IBM with the speed of 5.2Ghz, but is not available to the public.
So, go with at least a Duo or Quad CPU to get the need for speed. This is especially true if you are doing several things at the same time as mentioned earlier. If your computer use is such that you usually do one thing at a time, a single P4 CPU will be just fine. The P4 is fine at multitasking as long as it is not simultaneous use of programs. Of course, if you are gaming, the duo or quad CPU's are your best bet, along with hefty amounts of RAM and a fast video card. Owning a duo or quad CPU will also mean much faster boot-up time, almost twice as fast many times.