@Rob
I tried this last year when I saw a post about PCI to PCI-E adaptor, I actually bought one and tried 2 different Graphics cards on my A1XEG4
one was Radeon X1650SE and the other was Sapphire Radeon HD4350
I tried them both with and without my existing graphics card connected.
essentially the computer would not boot further than Uboot. But no damage was done and the machine just baulked at Uboot.
The consensus at ANT was that Uboot needed to be able to recognise the PCI-PCIE card. I did comment elsewhere that an update to uboot for the A1XE would be helpful
not a technically professional analysis but then I am not a coder.