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.

congrat. on your new computer,
im also looking for new pc hardware, yes with quad core processor(maybe Phenom). currently work on my 2 years old pc (Athlon x2 3600, 2gb ram) 12 hour rander time is a pain!