I think a workstation with two xeon processors (dual core or not) can utilize both physical processors for applications whether or not the program is designed to be multithreaded (can run on multiple cores/processors). This is because the load balancing between the two xeon processors is handled on the motherboard.
Both cores of a desktop grade dual core processor may not be able to be used if the program isn't multithreaded.
general examples:
Non-Multithreaded programs on a core 2 duo vs two single core xeons will be faster on the xeons, since 2 processors can be used.
or
Non-Multithreaded program program on a core 2 duo vs dual core xeon (single processor), will have the same performance, each using 1 core.
or
Multithreaded program on a core 2 duo vs two single core xeons of the same speed will have similar results. xeons will have a slight edge since they are actually two separate processors.
or
Multithreaded program on a quad core 2 duo vs two quad core xeons. the desktop will use 4 processors, while the workstation will use 8 processors.
That's the other advantage of the xeons is that you can have two quad core xeons. They dont make desktop motherboards to use two quad core 2 duo. So you can generally have more processors if you're willing to pay the money. They also make 64 bit xeon processors (itanium) while they dont have 64 bit desktop processors yet. Therefore if you can load up on a buttload of ram versus the 4gigs max of your typical 32 bit windows xp desktop.
Most trading programs aren't super number crunching intensive unless you have many windows/charts open with a bunch of strategies. Strategy optimization processes can be greatly sped up with multiple cores/processors.
sorry for the rambling. hope that helps more and doesn't add to the confusion. Bottom line, if you arent willing to pay the money for the extra physical processor and/or more ram, buy the regular desktop instead of hte workstation.