I've been running an nVidia 8800GTX off of an Enermax 620Watt PSU for the last 3 years. No problems at all.
I recently bought an open box ATI HD 4890 Vapor-X from Newegg.
I uninstalled the drivers for the 8800GTX and used Drive Sweeper to make sure any remnants were gone.
I turned off the computer, disconnected the psu from the outlet, pushed the power button to dissipate any remaining electricity in the system. I removed the 8800GTX and installed the 4890.
I booted and it looked ok. I loaded windows and tried to install the 9.12 Catalyst drivers. It had an error and the event log said that the video driver had failed to load. I rebooted and it installed the driver the second time. It worked! I tried to run the Trial version of 3dMark Vantage, but it quit part way through ... because the 4890 doesn't have PhysX? That's what the error code looked like.
I was going to run a few more tests, but I wanted to reboot, to make sure the drivers were good.
I was greeted with a scrambled boot screen / post. Wha?! I was then also greeted with a BSOD saything that there was something wrong with the video drivers for Windows.
I shut down, reseated the 4890, and it was clear. I rebooted again and it wasn't. I then resorted to putting in my old 8800GTX and the bios / post looked fine. I loaded windows and reinstalled my drivers. Everything looks great.