dual core means it can run two threads in parrallel rather than swapping liek the pentium 4 HT's so effectively you PC can do 2 things at once, they've just released a 4 core processor two.
Normally the dual cores / multi cores are slower than a single core processor...

now.. what's tricky is deciding if you need one or not... this depends on what you use it for...

If you only ever use a PC for MSN and internet, save your money and but a mid range processor... if you use it for gaming or something similar, try a old HT rather than a dual core. if you often play games while listenting to songs on Itunes, try a dual core...