I would suggest you buy a new 64-bit system, making sure it is upgradable, and send me the dually.
From what I've seen, a dual system is equivalent to only about 1.5-1.7 times the speed of the chips. So, a dual 2400, would be equal to about a 3800, as long as you were running programs to take advantage of the dual CPUs. If you are only running one big program at a time, it will usually only run on one CPU. You get the advantage if you run programs specific'ly written to take advantage of SMP, or run a number of programs, and the OS splits them up to run on several CPUs.
Of course, the dually has limited upgradability. The 64 could be upgradable for years, as long as they keep the socket stable for awhile. Programs and OSes will also become optimized to the 64 as time goes by, so it will run better.