When you add a PCI-E or AGP VGA card into a slot on your motherboard the BIOS will almost always disable the onboard VGA; so the "...separate VGA socket higher up the tower..." will stop outputting a VGA signal and any screen attached screen will probably display a "no input signal" error.
Originally Posted by access-mdb
Both your screens need to be cabled to the HD4300/4500 VGA card. One of your schreens will probably connect via a standard VGA ("analog") cable (blue end), but the second will need an "analog/DVI" adapter if the screen does not support DVI. However, if the screen supports DVI there will be a corresponding DVI input port on the screen and the screen should have been supplied along with a DVI connector cable (white end-connectors).
Once you have both screens connected to the HD4300/4500 VGA card you should be able to adjust your settings in Control Panel/Display to suit your needs.
I should have reported back, I now have two monitors and have used a vga to dvi plug to connect one of them. I've got them set up to my liking via control panel. It would have helped if the user manual had explained this!