there are quite a few reasions, ranging from the z600 was not designed to have a 350 watt thermal load from the video card (plus your other items cpu, ram, drives) while there may be pwr supply adapters i don't know or trust that they are rated for the load you are going to place on it, the power supply needs to be a quality unit, most likely a single rail design
or you may have issues
It's your equipment, feel free to do as you wish, but i personally would not risk a 800.00 card
as i pointed ot in my previous post just because you can do something,...... does not mean you should.