The power supply unit provides your computer and all of its accessories with power. Choosing the right power supply unit for your computer depends on how much power you actually need. Here is power supply calculator that is very useful to determine the right PSU Wattage needed for your particular set-up based on CPU, Motherboard, Video Graphics Card (VGA, Memory, optical Disk Drive (ODD), and HardDrive (HD). I would recommend getting a higher Wattage PSU than recommend to incorporate other devices that may need more power (such as other PCI devices, external devices, USB/Firewire devices, cooling fans etc.).
For my PSU I purchased a XION PowerReal AXP-850R14N 850W Power Supply. This may be an overshoot in wattage, but it was a good price ($89.49). It has also worked really well so far.
