First, thnx for replying Sajuuk - I hope I can clarify things

1. Oh yes, I tried many driver versions/uninstalls/reinstalls - no difference. I never experienced any performance problems only the blue screen crashes.

2. 2D/3D - as I understand it ATI Tray Tools indicates the normal desktop mode of my 4850 has clocks of 501:750. Full screen 3D mode is 625:1000. I understand the ATI driver detects full screen 3d mode and switches between the two (but windowed games do not switch). In my case WOW is the only game where fullscreen doesn't trigger the card into 3D mode. ( I have to force it using ATT) (I don't mind doing it manually but thought it might be an indication of an underlying problem) - the brand new install of my machine+wow+drivers also does not switch automatically so maybe this is simply a WOW thing!
3D mode (625:1000) increases FPS by 10-20 and WOW plays noticibly better/smoother

3. I now have both monitors on the 4850 (have removed 2nd GPU)
......running 2 WOWs - 3D mode significantly increases FPS - so far no crashes this way but no dual box s/w installed and have only played a short time.
......Installed Keyclone, running 2 WOWs - 3D mode significantly increases FPS - crashed after 10 mins playtime (not bluescreen however, just noise & black screen)

Summary:
Happy with two monitors off the 4850
Yes, there is a difference between 2D and 3D modes

BUT still get crash when trying to dual box -