Well who makes the PSU?
Either would be fine. I run my pc off of a 330watt true power psu from Antec (specs in sig)
You will not need more than 330 watts for that system (from a good make) but if you get anouther harddrive, than you would.
Is the PSU a 24 pin one, as than it may be worth getting the 550 watt so you can use it in your next system, but if it isn't than the 480 will still be alot more than ample.
Cisco CCNA, Comptia A+, 1/3 through CCNP