It's not so much how many watts your PS has as it is how many amps it can give on each rail, especially the 12-volt rail for the video card since all the fans and drive motors use this as well.
I've seen plenty of 500-watt power supplies that maxed out at 18-19 amps for the 12v rail. Check the sticker on it or do a search for your PS specs on Google.
Hope you enjoy your new X1650 Pro, dude. It's like having an entirely new computer when you're gaming. SH3 with GWX 2.0 looks amazing!