Best power supply upgrades for Dell Inspiron 531

Hi there;

My name is Peter, I own a Dell Inspiron 531 which has a factory AMD Athlon 64 X2 Dual Core Processor 5600 of 2.81 Ghz with 3 GB of memory RAM. It has one DELL DVDRW drive and it has an empty drive for another CD RW or Blue ray Drive. The chipset is Nvidia MCP61 with an integrated NVIDIA GEFORCE 6150SE nforce 430. Based on your experience If I buy an NVIDIA Geforce 9800 and a Blu-ray R/W drive:

Which power supply would the best? 400 watt? 500 watt?, or 600 watt?
 

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