I don't even...
First of all, after looking at the benchmarks, I can confirm that the 5450 is
slower then 4550 (
source).
Second of all, the 5450 consumes
less power then 4550 (
source). So if that 4550 worked fine in your system we can rule out the power supply being the problem. And digging even further in to the problem and what you said in your initial post, no it does not sound like PSU issue. All the cards you've tried consumes very little power while iddle so 250-300 watts should be enough to at least boot in to desktop.
I'd look at that bios update first. There's no different 32bit and 64bit versions of the update, just one which works for both operating systems (it's irrelevant which operating system you have). If it's some software you run inside your OS to update it, 32bit can be ran in 64bit OS (you do this all the time with things like internet explorer and stuff).
And before you buy 3rd card, please post back with budget so we can recommend you something that's actually useful.
One more thing;
You sure you saved the settings before restarting? (usually f10 to save and restart) I'm grasping at straws here though, the auto setting should work.