Directly powering USB ports from 5V rail

Hey. I have an OEM dell PC. Its problem is that the ports tend to just turn off when connecting a camera or other device that draws more than 300mA. I want to connect the USB power directly to psu's 5V rail. Is that a good idea? Do i have to connect and and 5V or just the 5V wire? I guess that it depends on USB controller.