Remember that people aren't about to pay $150/month in electric bills for their computers anytime in the next decade. The goal of hardware builders isn't just to make faster products and to not care about how much power they consume - Hardware companies also try to minimize the amount of power that they consume.
As an example, future Dual Core Intel processors based on the 45nm manufacturing process are expected to consume 57W, and AMD's K8L Dual Cores are supposed to range anywhere from 35 to 89W, depending on the clockspeed. This is down from the minimum 65W that Core 2s consume right now, and the 80-something Watts that Athlon 64 X2s consume. So its not like power consumption is going to keep increasing forever and never be stopped.
Just an example of power consumption actually reducing over time. So it might very well be the case that even 650W will never even come close to being fully used, and you could've done with a much smaller PSU. A 500W PSU still leaves quite a bit of room for any future upgrades; Also note that any "upgrades" to the amount of power an 8800GTX consumes would have to come in the form of ridiculously impractical hardware (such as 4x4).