1. Well, you never said what your normal motherboard was. So we cant tell you if they are compatible.
2. Since you said "mobile" processor, I am going to assume its a laptop processor. And if it is already in the laptop, then theres a very good chance you cannot take it out and put it into your desktop.
I recommend using that 4000+, make sure you have 2gb of RAM and buy a $100 dollar 7600gt. doing all that would keep you floating a while longer for the least amount of cash
I believe it will, as long as the voltage requirement is satisfied by the motherboard.
After looking around (Google) people are saying no and yes.. SO to be safe I think you should just keep your 3400, OC it, or uprade to a cheap dual core if your S939.