I don't go under the manufacturer's recommended number, if for no other reason than warranty compliance.http://www.guru3d.com/articles_pages/geforce_gts_450_review_roundup,15.html
Recommended Power Supply
Here is Guru3D's power supply recommendation:
GeForce GTS 450
On your average system the card requires you to have a 500 Watt power supply unit.
GeForce GTS 450 in SLI
A second card requires you to add another ~100 Watts. You'll need a 600 Watt power supply unit.
If you are going to overclock massively, then we do recommend you purchase something with some more stamina.
keep in mind this review was written in 2010 .... use of a modern 2013 era CPU will reduce that system's power needs.