The GPU does take quite a bit of power, however, when the PSU fails it generally fails to a degree that nothing works properly. I've never had a PSU 'die down' and lose only 50-100 watts. It's always been complete failure. Also, when will your new GPU get there? And the last thing I can recommend is run benchmark tests for other parts of your computer, if it only crashes for the GPU and not, say, the processor then you know for sure.