You are looking at Opterons and Xeons. The P4 is not a dual CPU design.
HOWEVER, the HT version of the P4 does show dual CPUs in Windows NT, 2000, XP and Linux. And does pretty darn good in transcoding sessions at a good price compared to dual CPU Opteron/Xeon based computers.
Today, I'd get a 3+GHz P4 with 800MHz FSB, later this year a dual core Athlon 64.
I would like to build a dual processor computer but I don't know what motherboards can support this and what processors work best. I like to multi task and my computer now is just SLOW when i'm doing anything that's processor intensive such as encoding a DVD.
I read about multi-core processors and if they work the same as dual processor then I suppose I could use that.