How much power supply do i need ?

[Red Devil]

Specs:

Intel Core 2 Quad Q6600 2.4GHz
ASUS P5Q Pro LGA 775 Intel P45 ATX Intel Motherboard
Seagate Barracuda 7200.11 500GB Sata
MSI Radeon RX HD 3870 512MB 256-bit GDDR4
Kingston 2x2GB Ram DDR2 667MHz
Toshiba CD/DVD Writer
Creative Sound Blaster

I have a power supply of 400 watts but is that sufficient ??

 

[geofelt]

If it is a good quality psu, then maybe. Most likely not.
http://ati.amd.com/products/Radeonhd3800/requirements.html
The amps on the 12v rails is the key measurement. I think 26a is recommended for a 3870 system.
Cheap psu's deliver watts elsewhere, not 12v, and they do not deliver advertised wattage at higher than room temperatures.
What psu do you have?
List the specs on it's label.

Look for a unit from PC P&C, Corsair, & Seasonic.
Here is about the minimum I would buy: http://www.newegg.com/Product/Product.aspx?Item=N82E16817139003

 

[sp0tswood]

You can use this to calculate your load.

 

[delluser1]

You can use this to calculate your load.

Link doesn't work.
Just to keep it real, the free version of the Extreme PSU calculator doesn't calculate the load, it provides a psu wattage recommendation that is based on the max power draw of the components entered and a good bit of overhead. Don't know about the pay version.

This free calculator is pretty good at letting you know what kind of load you'll be looking at during a gaming session
http://web.aanet.com.au/SnooP/psucalc.php