Re: Intel processors
The Core Duo processors have dual core's -- it's like having 2 processors on one die. This is better for multi-tasking. Unfortunately, like most people, maybe you were taken in by the "GHz is everything" myth? This is also why you think you've "had a better life with Intel". Have you ever built an AMD system? AMD exploded the GHz myth long ago with the Athlon line of processors, and then since 2004, the Athlon 64 processors. GHz isn't everything by any means, that's just something Intel would have liked you to believe before they changed their strategy and started labeling their processors in a way that wasn't based on clock speed alone (they took a leaf out of AMD's book, basically). With AMD, a "slower" chip may seem less desirable to the average person, but AMD chips can perform more instructions per second -- the clock speed doesn't actually mean much, that's why my 2.0GHz Athlon 64 blows away a Pentium 4 3.2GHz (and higher) without any trouble at all.
If you're building a new PC, what you should be looking at is the fastest chip you can afford. Intel finally got out of the dark ages recently after being slaughtered (price and performance-wise) for the last few years by AMD. Intel's newer chips (made after the Pentium 4) have been great, so you can go ahead and safely buy anything produced recently and it will perform much better than any P4. I'd look at the E6600 and processors around that price (if I was looking for an Intel chip). You can get one for just over £200 in the UK. If that's too expensive, have a look at the E6400 at around £185, though I think the E6600 is worth it for the extra 30 quid or so. Below that, the E6300 is pretty good. Get the best you can afford.
Shame you've ruled out AMD, I must say. Due to their price drops recently there's some serious bargains to be had, you can build a wickedly fast AMD system for not much money. But despite my leanings towards AMD, the Intel Core Due is a better choice for now. Once AMD release their K8L line of processors, AMD will be back on track. They do have some very nice dual cores out nowadays, though.