I was using a ATI-3450 PCI-e display card very well. I recently bought a new upgrade card from powercolor ATI-5450 512MB-DDR2. It even failed to POST in my mobo (ASUS A8N-VM CSM). The new card has been verified with another mobo flawless. I have upgraded the BIOS of my mobo to the latest one 1007. The PSU is 450W and the display itself have no need to apply any additional power supply and it's passive cooling design as well. What can I do?
It also sounds like a BIOS issue to me.
Unfortunately, Your Motherboard was released over four years ago and is a dead platform (socket 939).
As such, I highly doubt ASUS will be releasing any further BIOS updates (the last update was in January 2007).
I would recommend returning your new 5450 and saving your upgrade budget for a full platform upgrade.