Dan dual cores are better than single ones in that its effectively 2 single processors on one chip.
Windows sees them as 2 processors, and can assign each one different tasks, which makes them ideal for multi-tasking (e.g. converting some video whilst surfing the net etc. etc.)
Some applications also take advantage of the dual cores and can use both of them to process what they need to (this would make your dual 1.6Ghz cores better than the 1.8Ghz single core) since they would use effectively 3.2Ghz of processing power.
There are thousands of articles available on the net that might make a stronger case than I have :D but it is worth it.
A good place to see CPU benchmarks though is:
www.tomshardware.co.uk/cpu/charts.html
And you`ll see the difference the Intel D series (dual core) makes there.
HTH
Miles
I believe Windows Vista is optimised to work with dual core machines.
If I were buying a new computer I`d only go for dual core these days.