You didn't though, you assumed that the recommended power supply was how much power the GPU consumed not what was recommended for the system as a whole... He is correct that the 7750 uses 55W, the 6570 uses 50, and the 6450 uses 31W, that is their designed power, that is how much the card itself uses.
Most of your low profile cards are going to be around that power level, without a PCI-E connector the card only has access to 75W. Something like a 384 core GT 630 would be a massive upgrade from your 8400 GS while consuming very little power ~25W
http://www.newegg.com/Product/Product.aspx?Item=N82E16814121778
http://www.geforce.com/hardware/desktop-gpus/geforce-gt-630/specifications