I am looking to upgrade my CPU form an Athlon 64 3500 socket 939 to an Athlon 64 X2 4200 socket 939.
The available CPU's I am finding on the market are all OEM - No cooling fan included.
Does anyone know if I can safely use the same heatsink & fan from my 3500 on the 4200?
Your heatsink will work fine. If you overclock the new dual core, an aftermarket cooler may be an option for you. Also, the 4200 dual core will draw more 12v power than your 3500. Make sure your current psu remains stable and otherwise does not cause excessive heat or system crashes. HTH.