Want a long answer? Too bad:P
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.