Well I just bought Club3D 8800 GTS 512 MB G92 PCI-E.
And it just hangs on both Windows XP and Windows 7 RC 64bit, but not in safe mode.
Tried various of different drivers (newest, 16x, 181 etc) and still works fine on safe mode, but hangs on Windows loading screen or awhile after (1-2mins).
Have a 650W psu and yes it is new. My old HD 2900 Pro 1024MB works just fine on both XP and 7 RC.
It is possible the old ATI drivers were not fully removed.
Make sure they are uninstalled then download either Driver Cleaner or Driver Sweeper and install.
SET A RESTORE POINT.
Restart in Safe Mode and run the driver cleaner software, but examine evey item it is going to delete and deselect any that look wrong before clicking the 'clean' button.
Restart and install the correct Nvidia software.
Post back with results.
Set restore point. Went to safe mode -> runned the Driver Sweeper and deleted ATI stuff. Restarted and installed the newest nvidia drivers and after boot it hangs before the log in window. Mouse appears and then the sound appears as it should, but then hangs with mouse on the black screen and the mouse is not moving of course.