Dell e520 power supply help

I currently have a dell E520 (2.66ghz duo core) other specs here: http://www.dealtime.com/Dell-Dell-E520-Desktop-Computer...

I want to upgrade my power supply to get a better graphics card then the one I have (visiontek 4550). I recently contacted dell and asked about a new power supply then the current 305 watt one I have stock in the system. I was informed by dell that I was unable to upgrade the power supply. I have never heard of anyone say you couldn't upgrade the power supply before so I am asking if the rep is right or if I can get a bigger power supply?