A CPU will literally just die not start "dying", you can tell if a CPU is dead by if it says so in the bios you have no cpu. And yes does sound like a HDD, all he needs is a 500gb or 320 with 7200 RPM drive and install windows 7. and maybe you could tweak his system with a new PSU and a geforce 9800gt for that editing plus HD videos and if he plans to game laster
No don't upgrade to ddr3 there is no need, because if he is basic user and not a gamer he doesn't need ddr3 ddr2 is fine. Random Acess Memory is calculated in speed by MHZ the higher the MHZ is the faster the module is. 800 MHZ ddr2 ram is the best for him I would give him no more than 3 or 4 gigs of ram for his needs.