I recently upgraded my computer from:

  Intel 2.4 GHz P4 w/HT and 2 GB of RAM

to

  AMD X2 4800+ 2.2 GHz CPU and 4 GB of RAM

as I expected when I did my research, most single-CPU applications run about 5% slower on the new system.  However, everything that supports both cores runs just over 200% faster.

Just a note that if you are doing a similar upgrade, be aware that in 32-bit Windows your apps may not run faster if they don't support the newer hardware.

I was limited in funds when I did the upgrade which is why I only got the 4800+ processor.  I just ordered a Phenom quad-core processor now that I have some more cash available.  The primary reason for upgrading was to get into something that WAS upgradeable, my old system had hit its peak.