Looked at the Nvidia spec page for that GTX 480 (quoted below). Given the specs in the PSU image above, you will be not have sufficient power for that card. Especially when you consider other components which run from the 12-volt rails. Either stick with the 550ti, or get a new PSU. A complete list of your system components will be helpful in giving best advise.
"Geforce GTX 480
Minimum of a 600 Watt power supply with a combined +12 Volt current rating of 42 Amps
An available 6-pin PCI-E power connector and an available 8 pin PCI-E power connector"
http://nvidia.custhelp.com/app/answers/detail/a_id/2587/~/what-are-the-power-requirements-for-geforce-gtx-480-and-the-geforce-gtx-470