Well, it depends on what will you be doing with the pc. If you area gamer, then you are better off with overclocked opty single core, but if you are wanting to transcode movies, burn, office, etc at the same time, then get x2 3800, since it is the best bang for the buck.
As i said, Opteron 144 is on sale for 134$ US at mwave
and you can find the cheapest x2 3800 for 156$ US at
www.onsale.com
It all depends what you want out of your machine