I'd also stay away from the Pentium Ds--that was first generation dual cores out of Intel and they ran really hot--I have one and its a good CPU, but it runs very hot--in fact, I water cool it. Intel has new dual cores (core duo) but I don't know abou those--maybe someone else does
The AMD dual cores run much cooler and are the best bang for your buck right now in terms of older technology dual core processors. I have an AMD dual core machine as well.
I would recommend dual core for the average user because you'd only want single core if you needed to a processor that had a much higher clock speed for the same money--obviously, you are going to pay more for a 3 Ghz dual core than a single core so you could buy a faster single core processor for the same money.
If you are looking for a new processor, I'd recommend AMD dual core if you are concerned with cost--if not, then maybe someone else can talk about the new Pentiums, which are considerably more costly.