well, if it has been running superb before and nothing else has changed in the system
the heat is likely caused by one or few of following :
-Thermal paste dried up, evaporated or missing after reinstalling heat sink
-misalignment of heat sink when reinstalling it
-Dust within the heat sink fins *
-Dust in the case air intakes and outlets
-Dust in the power supply air intake and outlet
-Dust in the VGA card cooler/HSF
-minor overvolting of CPU due to substandard power supply
-thermal paste dried up under NB and SB heat sink
*) Usually cannot see the amount of dust within the CPU heat sink,
remove heat sink, remove its fan. Then rinse the heat sink with water.
reinstall, fan, heat sink with fresh thermal paste.
Hope this helps. Good luck!