I'm getting very frustrated with this now.
I just tried removing both drivers and ATI's install manager, CCC, PowerExpress... any software relating to ATI so that I could reinstall it from scratch with some of torrentg's suggestions. Well, after removing the driver (half way through the ATI software uninstall) my screen went black, which makes sense if I've taken away the hardware that makes the screen colourful and pretty doesn't it, so yes, I'm an idiot for removing both at once. On the other hand, I took the power away and rebooted (it came on in 640 x 480) and Windows started trying (and succeeded) to install the cards. It seems that both card drivers are now working, as no error is reported. I have disabled the 3200 display adapter in device manager and still get windows aero in 1600 x 900 resolution so I guess it really WAS working? But then I enabled the 3200 again and the screen went to back to black... but on rebooting they're both working!!?!? PowerExpress is NOT running.
I reckon this is a problem with switching between the two cards as torrentg suggested with updating motherboard drivers. Otherwise it wouldn't have worked until I enabled the other one, would it? Unless it doesn't work at all, and it was just Windows (or me) being stupid.
But I don't actually know what I'm talking about, so I can only hope this stuff helps for someone who does know what they're doing.