Hello Alf
Kontakt 2 is compatible with Dual Core Processors; also most (But not all) other VST instruments have also been upgraded to allow Dual Core Support.
Computer Component Suggestions
Most modern Motherboards will accept both single and Dual Core processors, and therefore I would suggest you go for a single core of at least 3 Ghz+ for starters, and then when you have gained more experience you can go for a Dual core CPU. (By this time the dual core CPU's will have increased considerably in speed and also their prices will have dropped significantly)
Go for at least 2 GB of DDR2 Ram (2 x 1 GB sticks will be fine) and make sure you have at least 2 Ram slots free.
Go for the largest hard disk you can sensibly afford, the speed should be 7200 rpm + , a 16mb + buffer and a SATA2 interface, if you can afford 2 (Smaller size if necessary) then do so, and set them up as a Raid 0 Array. (The motherboard will need to support this)
A high quality sound card is vitally important.
As far as graphics go, a medium quality card will be fine.
Finally, keep the computer just for music, and optimise it specifically for music.
Tip. If you are building your own computer then purchase an OEM version of Windows, (Assuming you decide to follow the Windows route) as this will be considerably cheaper then a Retail version.
Hope this helps

Bill