If you are gaming, and playing a game that does not use the Quake 3 engine (that is the only gaming engine that is SMP, right?) then when you game, you will only be using one CPU.
However, the load of the OS will equally distributed (most of the time) across both CPUs, therefore, XP uses ~50% of the CPU on a single system or ~25% of each CPU on a dual system.
(Those percentages are presented for illustrative purposes and are not intended to be accurate except when relative to each other)
So, even if the program you choose to use is not SMP, you still might see
a benefit in a dual rig. The non-SMP program will not run faster, but you will be able to multi-task much better with a decreased chance of over tasking your CPU.
Besides, Photoshop, Premiere and other programs like that are already SMP enabled.
If you are penny pinching when building a machine, then you should probably opt for a high-end single CPU. But if a few hundred extra (yes, one extra chip and about $100 extra for a dual board) won't break you, then definately consider it.
Also, some people use Durons because some of them are stable in dual rigs and they are cheaper. However, most people just opt for the MP line from AMD.
Hope that helps.
By the way, for more information
http://forums.2cpu.com