Well my 8800GT arrived yesterday and fitted into my "small" case without a problem (I was concerned that it would be too long but there's plenty of clearance).
First impressions with HL2 weren't good - a lot of flickering textures, solved by installing the latest 169.02 version.
Performance - superb...at 1680x1050 it's as smooth as a very smooth thing covered in virgin olive oil
Noise - loud at startup but then drops down so it's quieter than my old 7900GT
Temps - hovers around 55 at idle, rising to 77 after some intensive gaming (this should drop when I put it into the P900 with the side panel fan right next to it).
Stress test using HL2 Lost Coast gave 150fps...I think I can live with that....
Anyway, that's all for now....I have some more "testing" to do