Quick update:

I've started the trial of IS Boxer - a little bit harder to get the hang of initially than Keyclone but I love it! Will definitely take it up full time. (thnx to all those posting hints/tricks & guides)

Anyway with IS Boxer, I have OC CPU to 3.2GHz, both monitors on the 4850 AND ATT manually switching the GPU to 680:1010 - not a single crash! Main char on Ultra & getting >60 fps so I'm happy !

So, whether Keyclone had some weird interaction with ATT (how?) I don't know but (touch wood) looks like a year of bluescreens is finally over !

Thnx all for posts on dual GPUs, dual monitors, W7, Keyclone & ISBoxer.