I would be interested to see the sticker and now much 12 volt amps(combined for multi rail power supplies)
In general any decent 350(even 300 or lower) watt power supply in a non overclocked system will run a 7750 like a dream.
If it makes you feel better I am running a 650ti(and a 5770 before that) on a 300 watt FSP(22 amps @ 12 volts) unit
. Those both suck more power then your card.