You are double posting....and again, this is a relevant question, as there are many, many dual core CPU's that run at speeds a lot higher than 1.9ghz. Single core CPU's are a thing of the past, if you are buying or building a new PC, buying an old outdated single core CPU is a horrid mistake.
The entire point of dual cores is that you can have the same speed at half the freq. and thereby save power.
Dual cores potentially draw less power, but that is far from the 'entire point' of a dual core.
From a marketing perspective, the chip makers want to offer you something faster, with the hopes that you will buy it. As there haven't been any major breakthroughs in increasing clock speeds in the past couple of generations, one way to overcome this 'lack of progress' is to package more cores on a die.
The modern Dual-Core CPUs have a much faster architecture to them. A single core from the modern Dual-Core CPUs will probably outperform a single core CPU of the same clock speed anyhow.
With the newer Dual-Core CPUs though, you can perform tasks with greater ease without having as high of clock speeds.
Keep in mind, if you are planning to rebuild your system, don't just get any old dual-core cpu. The Athlon X2 and Athlon 64 series are quickly becoming outdated though. If you do any kind of PC gaming you're due for an upgrade beyond those.
Between the Athlon 64 2.2Ghz (single core) and the 64 X2 1.9Ghz (Dual-Core) I'd go with the dual core.