i'm not sure what you mean by additional power source.
some high-end video cards require direct hookup to the power supply. if thats what you mean well it really depends on which cards.. they all differ.
if you're wondering if your powersupply is sufficient, well that depends on how powerful a powersupply sony put into your comp. depending on which card you choose, they'll have a power requirement for it. so compare the power requirement to what you have, and if its not enough, you'll need to replace your powersupply.
"Is virtue a thing remote? I wish to be virtuous, and lo! Virtue is at hand." -Confucius