PSU Wattage For ATI HD3850 512mb

hi, guys (and girls)

2 months ago i bought an awesome ATI HD3850 (X turbo 3 version) graphics card from CEX (http://www.cex.co.uk/(which is like a 2nd hand store))
i've had problems with it on and off really. it works one day the the other it doesn't. now it doesn't work at all and my PC ,when i turn it on, does one long beep and 2 short beeps (where i've researched means a Video card error).

i took it back to CEX under the 12 month guarentee. theres this bloke there that had "supposively" tested my item
and he keeps telling me, even before he had look at the condition of the item, that it was all about my Power supply Wattage, that i need atleast 600 Watts to power it. but this is not what ATI recomends. they recommed 450 Watts!

in my Computer, i have:

550 watt PSU
An Amd 2.7Ghz black Edition processor
a 250 GB HDD (don't know the fequency)
a belkin Wifi desktop card
and a Disk drive

he told me that there was no hardware fault with the 3850 and that it's my PSU wattage. I came home and took out my Disk drive, Hard drive, Case LEDs and Wifi card and tried it again. still nothing happens appart from the SAME beeps. if the bloke was correct, the Graphics Card should boot and load my BIOS to tell me i have no HDD and disk drive plugged up to it.
Is this bloke Right and i need to get a 600 watt PSU, or is this bloke Corrupt and Doesn't want to take my Item Back?

any and all help appreciated