Graphics Card Problem

[Raggy]

Can anyone please help advise?

Had a problem where my PC would inexplicably switch off, either when running a performance test or when playing the graphics heavy Civilization V. After running www.memtest.org and finding no problems, we removed the NVIDIA GeForce GTX 550 Ti and ran the PC with the lower spec on-board graphics card. Though slower this ran fine, so we isolated the problem to the NVIDIA, which was just over a year old.

So I replaced the NVIDIA GeForce GTX 550 Ti (1GB ram size) with a new one (Powercolor AMD Radeon HD 7770 2GB GHz Edition Card). But now windows will not start up, other than in safe mode. It just gets stuck on the 'Windows Starting' screen and nothing happens.

Can anyone help explain please? Is it because the new card has an extra 1GB ram? Or is it something else? Running out of options! Any advice will be much appreciated.
I have the PC spec if it will help

[David (PassMark)]

Is your power supply adequate?
I assume you are running a 64bit O/S?
The problem is most likely driver related. It is unlikely that it is due to the amount of video RAM.

[Raggy]

David,
thanks for advice
My power supply is 500w. The manufacturer of the new AMD Radeon recommends 400w power supply, but on some other sites it suggests 500w. Either way, it should be ok for power supply.

I am running 64bit windows 7.
The AMD Radeon came with a driver CD but it doesn't seem to help. Someone else suggested installing latest drivers from their website so I will try that.