Just to add:
Even with XP Pro unless the app give special instructions to use the 2nd core it will not be used. If doesn't mean that if the first CPU is all used up for processing I do not believe the second core will be used (I maybe wrong about this) unless the app specifically can use it. The is a 64 bit version of XP that will use it as long as the apps support it. Most are web services and databses. So if you run server process such has HTTP, Java servers with Oracle connections then you will be using the Dual Core to its best ability. You won't be seeing any added performance until you hit something like 50,000k+ transactions per day. Low volumes will not see added performance. So if you had a single CPU single core vs Dual Core CPU at very high volume transaction hits and access that is only when you would see an advantage. I also know if you do 3D rendering programs like Maya and other of the such apps you will be rendering in the order of magnitude in seconds faster or minuted faster for really high rez 3D objects.
I am presuming WinXP Vista will harness the power of dual core...or so we hope.
http://techrepublic.com.com/2300-10877_11-6043696-2.html
