|Your power supply must have at least the minimum capacity required to support a system with the graphics card you are using installed, or the max graphics card you might install in the future.|
(Onboard video - video built into the mboard - IS NOT A CARD ! )
If that info is not in the ad for the video card, you can go to the video card maker's web site and look up the specs for the model - often under system requirements - the minimum PS wattage, and, more important, the minimum amperage the PS must supply at 12v is stated. If you don't find that, any card with the same video chipset including any letters after the model number has very similar minimum PS requirements. Some power supplies have two or more +12v ratings - in that case, add those ratings to determine the total +12v current capacity.
If you're a gamer...
In most if not all cases, the max capacity rating of the PS is an intermittent rating. It's recommended that you do not load your PS to any more that 80% of that rating if you are going to be using something that puts a constant load on it, such as playing a recent game for hours on end. In that case, you multiply the min capacity stated for the system with the particular video chipset on the card by 1.25 to find the min. capacity of the PS you should get.
If you need to get a PS with more capacity, you can usually replace it with any decent standard sized standard ATX PS.
Standard (PS/2) power supply size - 86mm high, 150mm wide, 140mm deep, or 3 3/8" h x 5 7/8" w x 5 1/2" d , or very close to that, though the depth can be more or less for some PSs.
Don't buy an el-cheapo PS.
See response 3 in this:
Minimum recommended PS capacity you must have for your system according to the specs for HD4650 ATI brand cards - 450 watts.
Most other manufacturer's cards with HD4650 - 400 watts.
If your power supply doesn't have enough capacity, the video card will often work anyway, but it is 100% loaded or nearly so all the time, and the PS will be slowly damaged over time, to the point where the PS will eventually not be able to properly work with the system when the video card is installed, and if the PS fails completely, it can damage things other than itself while failing, e,g. often, it fries the mboard.