I don't have an answer for you, just to say that isn't a conclusion I'd want to jump to.

I am guessing of course, but I have a feeling that's the exact reason my 5450 doesn't work. Mine is 512MB (I think!) and doesn't work when other working ones are maybe 256MB?

I've certainly seen it said about cards in the second slot and with PCI memory allocation and BAR sizes etc., I can believe it.

Hopefully someone can get you a specific answer.