T1600 with Nvidia GTX650 Power requirements.

I have acquired a Dell Precision T1600 which I intend to use purely for the Zwift cycling / training application. I have also acquired a graphics card as follows;

www.gigabyte.com/.../product-page.aspx

Gigabyte Nvidia Geforce GTX650

According to the specifications for this graphics card, a 400W System PSU is required;

www.geforce.co.uk/.../specifications

However, the above also says that the card has a maximum draw of 64W (I believe the card slot in the T1600 can supply up to 75W).

If I then go to the "eXtreme Power Supply Calculator";

outervision.com/power-supply-calculator

And enter all the system specifications including the new graphics card, the recommendation is 288W (the T1600 I have has the standard 265W PSU fitted)

So, do I need a 400W PSU, and if so, which do DELL recommend?