When it comes to buying PC components, the single most important one is the processor (CPU). For this reason, i feel it fair to dispell some of the beliefs that some people have with regard to dual core processors, which is not helped by some unscrupulous sellers who only add fuel to this misconception.
When AMD and Intel advertise their multi core processors, whether it be an AMD Athlon 64 X2, AMD Phenom, Intel Core i7, Intel Core 2 Duo / Quad or Intel Pentium D, they always show the speed of each core individually. For example, a Core 2 Duo E8400 is a 3ghz per core processor (also written as 3ghz x2). What they do NOT say is that this is a 6ghz processor, and for good reason. Dual core processors do NOT run in such a way that their two cores combine together to run a single application at the speed of both cores added together. In terms of sheer clock speed alone (Note: This is not how you should judge how good a processor is), the fastest dual core processor produced is the Pentium D 960, which runs at 3.6ghz per core (3.6ghz x2) but at no time at all does it ever run at 7.2ghz, yet there are still ebay sellers who are selling dual core CPU's or PC's with dual core processors installed, and describe them as running at anything between 5.2ghz to 7.2ghz. Once again, in terms of sheer clock speed alone, the fastest processor ever released was the Pentium 4 670, a single core processor, which ran at 3.8ghz. Neither Intel nor AMD have ever released a processor with a clock speed faster than 3.8ghz without being overclocked. Anyone stating they are selling a processor or PC which is running faster than 3.8ghz are either overclocking their processor (which means your warranty with the manufacturers of all your PC's internal components are void) or are misleading you.
This is not saying dual core processors are not worth buying, far from it, but they work in a specific way. If we take the Core 2 Duo E8400 as an example again, you can have the CPU using one core for your main task (which will be running at 3ghz) while the second core will be running your background tasks, such as anti-virus program, which will also be running at 3ghz. This is much more efficient than having a single core processor having to run everything by itself, which would result in the PC running slower than it would if you were using a dual core processor. Another way of using the dual core CPU is if you are using programs or games that are specifically written to use the dual core processor in a way that allows it to use one core to run one part of the program, while the other core runs another part. This is called a "multi-threaded" program.
The usefulness of a dual-core CPU is not in dispute, they are fast becoming invaluable as even basic programs such as CD burning software are becoming more complexed in their structure and being programmed to take advantage of 2 or more CPU cores. With the price of dual core CPU's dropping fast, there's really no reason to choose a single core CPU over dual core. But, be sure about what you are buying and don't think you are going to be getting something running 5ghz or faster. These days you simply cannot buy single core processors new. All new processors feature 2, 3, 4 or even 6 cores.
*UPDATE* This misleading advertising is still going strong, and with the advent of tri, quad and hex (6) core CPU's, it is, if anything, getting worse. There are now sellers listing multi-core CPU's and adding all cores together. With the fastest quad-core processor in terms of clock speed being the Intel Core i7 2600, which has 4 cores running at 3.4ghz each, they are describing it as a 13.6ghz CPU. Or the fastest hex-core processor (once again in terms of clock speed alone) being jointly held by the AMD Phenom II X6 1100T and Intel Core i7 980X, both have 6 cores running at 3.3ghz, again there are listings stating these processors as 19.8ghz CPU's .... which is absolutely ludicrous.