Cheers roonj, sorry for the jumbled nature of my posts, I'm just trying out everything that I can find.
The big problem here is that with the 4890 installed my BIOS doesn't see any of my (now) 3 HDDs so I can't even boot into safe mode. The other day I found a tool that help clear drivers completely so I used that and then installed the 4890 but the same happened, no HDD were found in BIOS!
I've also posted on the Asus forum to see if their techs could help, the only response I got was from a guy who had had the exact same problem whilst trying to install a ATI 4850! Maybe it's an inherent fault with this motherboard.
But it's not all doom and gloom, I have got it working. If I leave my 9600 in the primary PCIe slot but don't feed it power (leave the 6pin out), then install the 4890 in the bottom PCIe slot (third slot), it boots into window recognised my 4890 as the primary display device and runs games without a hitch (at least so far).
Ignoring the obvious stupidity of this set up there are two big problems with doing this. The first is obvious, I can't sell my 9600 so I'm around £40 out of pocket. Second, the 9600's fan spins at full speed constantly which is pretty loud.
I installed an older (8500gt) passively cooled geforce this morning but, as it didn't require extra power to run, my pc used it as the primary device! Tonight I'm going to see if I can get that configuration working. maybe if I deactivate it windows will default to the 4890.