Thanks for those replies, which were helpful. I still have my old monitor hanging around at the moment and was able to plug that in. The BIOS doesnt give the option to disable the onboard graphics, but it does enable me to reduce the amount of memory reserved for it to 1mb.
The effect of this is that the full 512mb is now reported in My Computer > Properties
and I have now disabled the Integrated and PCI Secondary monitors in Windows, so the signals arent going to those ports all the time.
The other thing the BIOS has let me do is change the priority of the monitors from Integrated > PCI to PCI > Integrated, and lengthen the amount of time the POST screen is displayed to let me hit F10 quick enough.
The thing is that instead of improving the performance of the computer (albeit I only expected the same or a slight improvement), since the PCI card was installed it seems to be running really slowly. Thats why Im thinking perhaps at sooner than Id planned I might look for a computer that will support this widescreen resolution out of the box.
Thanks again for all the help received