A computers performance does NOT go up faster by simply increasing the number of cores it has. A six core 2.4 Ghz machine is still a 2.4 Ghz machine and will not run standard apps faster than say a 3.8 Ghz single core machine. The 3.8 Ghz machine will be noticeably faster and will win out in performance tests again and again.
Only apps which are specifically written to be multi-threading for use with multi-core machines can take advantage of multi-core machines. It an app is not designed specifically to run on a multi-core machine, there will be no performance improvement running the app on a machine with a lower rated processor speed regardless of how many cores it has.
This has been written about extensively in the past, and I have proven this in person time and again. I have never had a single customer ever want to switch to a multi core machine with a lower processor speed than they already had once they had a chance to try one. The machine with the lower processor speed has always underperformed the machine with the higher processor speed, even in startup time and simple tasks such as opening apps and retrieving data via IE.
Multi-core machines are NOT designed to automatically parse out tasks to the various processors as you may have been imagining them to do. That's not how they work. An app has to be specifically written for multi-processor machines or there is no advantage to having such a machine. In fact, having a lower processor speed will only result in slower processing time, regardless of how many processors a machine has, as the additional processors simply aren't utilized by the OS or the apps themselves.
I've got two 3.6 ghz machines myself, one running XP, the other running windows 7, and my machines will kick any multi-core machines ass with a lower clock speed any day of the week, as I've done time and time again for many people in the past.