I am looking to buy a new graphic card- I mostly use it to watch HD videos.. When I look at the catalogues, most of the new ones list 400W power as one of the system requirement. I look at my PC's spec, and the power supply spec 250 W.
Does this mean I can't use the 400W graphic cards?
Are there still good graphic cards for my power supply spec?
Yes and no.
The GPUs that require a 400W PSU doesn't really need one per se, a quality 300W can run most of them with ease. But your 250W is probably a cheap/old unit, that can barely deliver 150W for the most part.
However. HD video isn't a tough task to do. A card like the 5450 can run of your PSU fine.
That's a general recommendation for those who don't really know anything about computer power consumption. You probably have a Dell or HP or other brand of PC and generally as long as you do not have a tri-core or quad core CPU, then the CPU power consumption should not be very large.
The HD 5570 uses about 19w of power under typical gaming conditional which most PCs with a 250w PSU can handle.
A more definitive answer can be provided if you list your computer's specs and as the various voltages and amps listed on the PSU's sticker.