Hi, first off, welcome to the forum
I was greeted well here, and aI'm sure you will be too
Now, the best system would be of course speed wise be the HP Pentium 4 2.8.
AutoCad being a 3d software application would need a good video card to get the most out of it, and a good processor would be nessecary also...
In my opinion, the best one for you would be the Dell Pentium 4 2.8ghz. I mean if you aren't going to use it for gaming, that'll definetly be fast enough, and even for gaming, it should be fine if it's with a good graphics card.
Either that or the Apple Ibook yet your getting more looks than features, and costs more... nah leave that one :P
If you wanted to go a notch higher, the HP Pentium 4 would be more than sufficent!
For most of the tasks you'll be doing though, a faster processor will not exactly give you much of an extra boost. Remember that when searching around, yet one lower than 1.0 nowadays wouldn't suffice for most tasks, and is best to look for more features
Hope this helps