Good article here that gives a good overview on Dual Core processors and those employing HT (Hyper-Threading).  I especially like the path the Microsoft has taken in it's licensing concerns with Dual Core processors.  Microsoft chooses to see that as one CPU in regards to licensing whereas some other companies have not gone that far yet.  Dual Core still being a bit pricey, it would pay to check with what software vendor you use to confirm the licensing requirements.  Seems to me that Dual-Core (will we see quad-core or higher in the future?) will be the way of the future.