PSU Advent T9106 upgrade 8800gt?

I've had this pc for about 3yrs now. I've upgraded the RAM from 512mb to 2gb and the graphic card from a 256mb 6200se to a 512mb 7600gs. I've also added a sound card. So far no problems, I have never upgraded the PSU...yet. I'm thinking of upgrading to a 512mb 8800gt for Gears of War. Will I need a new PSU? I don't know what my PSU is now. There's no sticker or marking inside. Tnx