Someone correct me if I'm wrong, but aren't the Pentium D's the really old Intel processors from like 2002-2005 or so? If memory serves, they're single core, but show up as dual since they're hyperthreaded. They ran so hot that Intel tried to push the new BTX case format for additional cooling, and the increasing heat from clocking single core processors higher and higher was the impetus behind the development of dual-core CPUs.
If this is the case, you can't really upgrade. You'd need a new everything except a case, and even then, today's components require more cooling than yesteryear's so even your case might not have sufficient airflow. You would need a new pc, basically, if I'm remembering right about the Pentium D's.
$600-$750 isn't much for a new PC. You say you don't know much about computers so I'd be hesitant to recommend building one, which most people will recommend. If you were willing to, you could probably find a good combo deal and stay within your price range, otherwise you might be looking at $800-$900 for something decent from a good prebuilt manufacturer like cyberpower.
If this is the case, you can't really upgrade. You'd need a new everything except a case, and even then, today's components require more cooling than yesteryear's so even your case might not have sufficient airflow. You would need a new pc, basically, if I'm remembering right about the Pentium D's.
$600-$750 isn't much for a new PC. You say you don't know much about computers so I'd be hesitant to recommend building one, which most people will recommend. If you were willing to, you could probably find a good combo deal and stay within your price range, otherwise you might be looking at $800-$900 for something decent from a good prebuilt manufacturer like cyberpower.