Everything went beautifully with the upgrade in physical terms.
UPDATE: Sadly after months of trying to diagnose the reboots I was getting I seem to have established that the cards cooler is pretty poor. I saw temps as high as 80oC in the case. I even modded a 120mm into the roof of the shuttle (looks so cool with blue LEDs) but I could not get it to stay cool.
I think it's only once ever got to the end of a run of 3D Mark and that was when I under clocked the card massively using the catalyst drivers sliders.It's pretty disapointing. You would have thought they would have designed the coolers to force the hot air out of the case and though the fins that way instead of having such as open design.
Even with the case off and all fans at 100% (it sounds like a jet engine) you still reboots.
The old GTX 275:
Without the GPU:
The new card (left) and the old (right):
Top down view:
The new card going in:
Everything back in the case:
PSU info (I doubt the Shuttle PC55 would make a difference as it not got the same efficiency rating):