Ok lets put this in perspective, my x850xt eats up more power then his
A x800 does not draw more power than a x1600pro...they are almost identical cards. Both run around 500mhz core and 1.1-1.2ghz for the mem speed, both are 128 bit cards. My guess is that the x1600 still pulls more power, due to its newer design...as ATI rarely goes down in power consumption.
*If you want to know for sure, please list the amperage off of the side of the PSU.
amperage*voltage = watts
If the PSU is capable of 350w total ouput, then you could pull [350w * 75%] = 262W before you start really straining the PSU. If its a crappy PSU, it will start straining with that load long before the 75% margin is crossed. Also...do you plan on OC'ing the GPU at all?
I dont doubt that it will work, just that its not a good idea to make it work. Your so close to the 'edge' of what that psu is capable of, you are just asking for something to go wrong.