I disagree. As the name implies, a single core processor is significantly more limited with regards to data processing than a multicore. This still applies to 'single threaded' software. A quad core processor and 8 gb of RAM make everything faster. Multithreading duties are often assigned by the OS (assumikng its 64 bit), so even if the compiler isn't multithreadng, your computer will have much more overhead available for all the other data.

The other problem is availablility. New single core processors are designed for efficiency in mobile devices, not speed. I think you'd have to find an old Pentium 4 or Xenon for a 'fast' single core processor.

So the short answer is yes, a higher performance PC will almost always make software run faster. In a laptop, look for an Intel Core Duo or the newer i3 or i5 processors. i7s are top of the line, but the cost to performace ratio may not be right yet. In a desktop, Intel Core 2 Quad processors are hard to beat when value and performance are considered.