My experience has been that the Xeon is significantly faster than the Pentium 4, but this is just anecdotal from how different machines have "felt" to me. It's probably worth a look for some benchmarks to compare the two. I wouldn't be too surprised if one of the 2.6GHz Xeons would come close to or even beat the 3.6 P4 in floating point operations (aka FLOPS) which are (AFAIK) the most important measure of CAD performance.
The dual processors really come in handy when running some combination of ArchiCAD, PlotMaker, BGArchiCAD, Word, Excel, Photoshop, etc. at the same time. I find it is not at all uncommon to have all of these going at once, and then there are e-mail, browser, and system processes going on as well.
I repeat that I am no expert in this area, but from what I've seen the experts have a pretty wide range of opinions about rating processor performance. In any case, either machine should serve you well assuming the rest of the specs are good. I would lean toward the dually myself.
