|Yes, basically what kuwese is saying is that not all your ram modules are the same speed. If you configure the pc to use all the ram, it can only run at the slowest of those two speeds.|
But when you configure it to use only 4gb ram, those modules with the slower speed are ignored - so the ram can now operate at the higher speed.
To answer your question about which configuration gives the better performance, I personally would prefer to use all the ram even if it runs slower than using only half of it. There's a vast difference between 4 and 8 GB.
Moreover, what's the point of having 64-bit Windows if you're not going to take advantage of it's ability to address more than 4 GB ram when you've paid for the extra modules?