What is the difference between these two GPU Coolers?

Arctic Cooling ATI silencer 4 Rev 2
and
Arctic Cooling ATI silencer 5 Rev 2

Also will either one of these work just as well as the other (performance and quietness etc)? Also will they both work with my X800GTO2?

I do not beleive they will both fit. According to the links on Newegg one is for the 420 core and the other is for the 430. The fan dimensions look to be different as well. Find out which gpu core you have... 420 or 430..

This should be the one you need. Says it is for all X800 series except aiw.