I don't think you need dual rails for a 8800 GT, but a 500 Watt won't cut it, I know that much. You need at least a 600 or 700 watt with around 30 amps on the 12V rail. I would, however, recommend you get one with dual rails to take the load off of the PSU..
I might have to recheck my sources. I heard somewhere dual rails weren't required, but highly recommended.