I can think of a few immediate causes involving the power supply. You got a lemon and the fan isn't working properly and overheats. Another possibility is that your PC is trying to pull more power than what your PSU can supply. How many watts is the new PSU?
Added: With this new revelation of an amber blinking power light, Dell says this:
- If the power light is blinking amber – The computer is receiving electrical power, but an internal power problem might exist.
- Ensure that the voltage selection switch is set to match the AC power at your location (if applicable).
- Ensure that the processor power cable is securely connected to the system board (see page 73).