The only way to limit the power a GPU draws is to never use it anywhere near full capacity.
As for that limitation you want to put on it, the reference cards only draw about 60W (75W tops for any vendor-OC'd models), & nVidia only recommends a 300W PSU as the minimum requirement. So, if your PSU is that low, an extra 10W won't really make much of a difference. Chances are you probably have an OEM-supplied PSU, which means you're lucky to even top 300W anyway (most are more in the 200-250W range), but getting a Bronze-certified 400W PSU won't set you back a whole lot.