1) yes 2)Depends how much you overclock 3)In the bios you up the voltage and change settings 4)No,
If you are having issues running Doom3 it definitly isnt the processor thats the problem area - its your video card. Your cpu should run Doom3 no problem. My 2.0 P4 runs COD2,HL2,BF1&2 with no problems. And most of those I run at max or close to max settings.
Heres my advice to you, dont mess with your puters cpu, its fine @ 3.0Ghz, save up and get a [email protected]
video card, youll immediatly notice that all of your games will be able to run @ max settings with little or no frame rate loss.
As for video cards get bare minimum 256 ram, preferrably 512 but as youll notice the price jump from one to the other is pretty severe. Then it all comes down to preferance, ATi or Nvidia.