1) more resources = (A) search the forums here (B) 2cpu.com
2) Games will run slower on a dually than the same speed single proc, so will processor intensive single programs
3) dualies are great if you are going to multitask
4) DISTRIBUTED computing <something different> dualies give you TWO processors and you only need ONE ethernet, power supply, video card, memory, etc. SO, if you're into folding or SETI ......... A dulie will complete WU slightly slower than 2 singles, BUT will take up MUCH less power (electricity) and in total less heat.
So if you're building a processor farm, dualies can do it for you more convenient / less space.