As bad bunny said - 2 cpu's unfortunatly do NOT give you twice the speed as having 1. Currently the only part of maya that is multithreaded (ie will make use of more than one cpu) is the renderer(including ipr and most of paintfx). Feedback inside viewports is actually marginally slower than a single cpu system, though not to any noticable degree Having said that, SMP systems are just so damn smooth that if you can afford one then you wont regret it!
One possibilty that may be worth looking at are the new 3.06 ghz Pentium 4's which have hyperthreading (gay name i know) which in simple terms makes one cpu appear as 2, though its still not as fast as having 2 physical cpu's, under the right apps it'll still provide a healthy speed boost.
Dont really know much about the different branded PC's, but a friend of mine bought a BOXX dual athlon machine and he's very happy with it