No idea, but it looks like you can leave the cage in so no drama.
290X has more stuff in the core so its naturally faster, the lightening is a custom designed PCB with a lot of features to overclock.
Now that the price has gone back up I guess looking at a cheaper 290 is a good idea.
The features the lightning has is aimed at pros.
Yep. thats fine, for that GFX card a 550W PSU will be ample, so if you can find a decent one for less, then sorted.
Example - http://www.overclockers.co.uk/showproduct.php?prodid=CA-003-SF