I need support please!!!!

Okay fellas check this out, my power supply i used on this machine and video was a 500 watt PSU antec smart power,and a 7800 gt...
the processor is a 3500 amd athlon 64,2 gbs of ram,sound audigy,and dvd roms...
ever since i took out the psu and 7800 and put my girls 500 watt ultra and tried to use a x700 pro..it stays at the motherboard screen never goes by it.Then i try a 6600 gt and it doesnt even show me a display..then i ll put the 7800 back in and it works but so slow to get in windows. Once i log in everything is fine! but this doesnt lol make any sense on my part. Im wondering if her PSU does su ck cuz her old machine did the same but i was using this 4 days ago,i mean this was my machine.
her PSU is a 500 ultra i swear that has to be it s but i need one of u guys to give me ur awesome input ,please!

 

How old is the PSU? There are ways to check if a psu works fine but it's a bit of a deligant job. Did you try your own psu again?