To summarise briefly, if I use DVI to desktop I get no image on the screen during boot, only once the nvidia drivers kick in do I see something on the scren (and this is at a low resolution 640x480). Xorg.0.log complains that the EDID information has a corrupt header. If I use the VGA cable with other computers (laptops and windows desktops) then everything is ok - I can right click the desktop and increase the resolution to 1920x1200). If I put a VGA->DVI converter on the VGA cable and connect this to my desktop I get problems again (I do see output during boot, but I'm still getting a corrupted EDID).
I think it may be the graphics card.