NOTE: This answer was written 8 years ago. Parallel programming has since become more relevant. This is likely due to inherent clock speed limits and the fast approaching transistor size limits.
Your primary problem is software not written for multi-core.
Look at Jeff Atwood's excellent article on Choosing Dual core or Quad Core.
for most software, you hit a point of diminishing returns very rapidly after two cores. In Quad-Core Desktops and Diminishing Returns, I questioned how effectively today's software can really use even four CPU cores, much less the inevitable eight and sixteen CPU cores we'll see a few years from now.
You are answered here (highlight copied from Jeff's article),
However, there were some surprises in here, such as Excel 2007, and the Lost Planet "concurrent operations" setting. It's possible software engineering will eventually advance to the point that clock speed matters less than parallelism. Or eventually it might be irrelevant, if we don't get to make the choice between faster clock speeds and more CPU cores. But in the meantime, clock speed wins most of the time. More CPU cores isn't automatically better. Typical users will be better off with the fastest possible dual-core CPU they can afford.
The issue of the Front-Side Bus (that term always amused me).
With Nehalem things change... as ArsTechnica said last year.
Moore's Law has given processor designers an embarrassment of transistor riches, and nowhere is that more apparent than in Intel's 45nm Nehalem processor. Debuting in 4- and 8-core variants later this year, Nehalem packs a ton of hardware into a single processor socket. (Early numbers put the transistor count of a quad-core Nehalem at 781 million; no numbers for the 8-core model have appeared yet.) But trying to feed all of that hardware with the Intel platform's existing frontside bus architecture would be folly. So, just as importantly, Nehalem also sounds the long-overdue death knell for Intel's positively geriatric frontside bus architecture.
The radical change in Intel's system bandwidth situation that Intel's new QuickPath Interconnect (QPI) represents is perhaps the largest single factor that shaped Nehalem's design. Between QuickPath and Nehalem's integrated memory controller, a Nehalem processor will have access to an unprecedented amount of aggregate bandwidth, especially in two- and four-socket implementations.
AMD moved the memory controller into the processor earlier and used Hypertransport.