As correctly pointed out the system will only draw what it needs. The only matter left is the matter of efficiency. PSUs are most efficient at 50-80%ish of their power output. In the case of your 1kW PSU, this would be around 500-800W of output. Lets pretend your tower consists of a 95W quad core CPU, 4GBs (2x2GBs) of ram, and the 5850. 95W + 50W (RAM, board, other) + 151 ~ 300W. Do you see a problem with this number? 300W < 500W, so you might not get that 80% number you were expecting. (a 500W would be 250-400W)
So, lets say 300W at 75% means your going to be pulling 400W from the wall. if you bought a high quality 500W machine that could do 82%, then you'd only be pulling 366W from the wall, saving you 34W. This ends up being a really small cost to you per day/month/year, but the lower PSU could be the better deal. On the same note if you get a great 1kW PSU that can do 82% at 300W, then there would be no difference. Or if you toss in another 150W GPU your 500W won't be able to stay at 82% and the 1kW might be the better PSU again. You need to know your load and the efficiency of the unit your looking at.
For the most part I say don't worry about it. 34W difference won't amount to much cost wise. Its more important you get a good quality unit that can handle what you'll throw at it.
And ,,, the mains draw is governed by the fuse inside the psu which is usually about 2amp or so for domestic applications,, the output WATTAGE is determined by the internal components of the particular psu..