Pci x16 express slot's power to video card

the 300w or 400w recommendation is just that a recommendation made by the video card maker to supply you with info on the approximate system needs for a system running that video card -- It will vary depending on how cautious the company is and how old the recommendations are - since there are a number of variables -- for example does the system use a 45w sempron or a 125w Phenom II CPU ? How many Ram Modules is the system using? How many HDDs and DVD drives? How many USB devices are getting powered ? How close to the rated Spec does the PSU actually perform (some cheap makes overstate the output by testing in a refridgerated room since cooler temps allow more output) ? etc.etc. -- so the GPU maker has to provide some sort of recommendation on what size PSU is needed but there are a lot of variables that will determine if your system needs more or less than that recommendation -- and to be safe you normally want to exceed that amount since PSUs will perform more efficiently and also last longer if you only use around 50-60% of its capacity since as units age the overall output declines and the closer to the max you are pushing it the faster the parts degrade.