General Power Supply Question

Obviously 650W.. It doesn't matter even if you have 1KW worth of PSU power.. The components will draw out power as per their needs and nothing more than that..

Ok thanks, I thought it would be like that but I just wanted to check as I'd rather not waste power but I'd like the option to put in a second graphics cards at a later point and so would rather take a larger power supply.

That's the correct approach while selecting the PSU.. Your current setup and your future upgrade plans.. In any case, its always a safe approach to choose a PSU from a quality brand with power rating which is little higher than what required by your current and/or future system plans..

To note, you can run two GTX460/5850s fin on a nice 650W, so make sure how many watt you need first.

Headroom is a good thing, more money in your wallet is better.

Very common question. No one seems to consider that his 400 - 800 watt (or so) PSU is plugged into a wall outlet that can supply 1800 to 2400 watts, and his PSU doesn't explode from that.  

Same principle.