They're about the same, but I'd go with the Phenom II 945 for AM3. The LGA 775 socket that the Q9400 uses is done for so I'd go AM3 for the future upgrade potential and the DDR3.
Message edited by Dustpuppy on 06-05-2009 at 06:18:23 AM
And stock coolers are fine at stock speed, but are not the quietest if if you ever do overclock then it would be best to change it. So no need to fork out money for an aftermarket one straight away.