Is Newer Much Faster

FWIW, Moore's observations-projections dealt with integrated circuit complexity vs manufacturing cost. Think number of transistors per chip which minimizes the cost per transistor. His first projection was doubling every year, latter he changed that to every two years. It was another Intel guy, David House, that came up with computing power (performance) doubling every 18 months. I don't know how well either prediction has matched reality. You could google for more info if you are interested.

Earlier in your post you mentioned buying a 3.6GHz P4 18 months ago, so I believe that would be a 560. Sharkey Extreme tracks retail CPU something that may be of interest:

10-20-04 --------- P4 560: $475

04-12-06 --------- P4 560J: $392 P4 660: $398 Pentium-D 830: $255 Pentium-D 840: $345 Pentium-D 940: $426

Each of the latter dual cores offers considerably more computing power than a 560-660, but you have to harness it. Much has been written on the subject of single core vs dual core performance and there are plenty of benchmarks which demonstrate which is better for what scenarios. A google search for said, looking for benchmarks that are relevant to the apps you'll be using would shed some light on things. Then again, you could just go ahead and buy what is in your price range and when you get the box use it for a day or two as you would your current box, keeping what best suits your usage pattern. For a small fee we won't tell your wife you are giving her the slow system ;-)