i7 3770 vs 3930 for Lightroom 4

I've never seen my ram usage go above 4 gigs on my 8 gig computer so how would 16gigs on my next compute improve its performance? Just wondering since so many folks seem to want ever more ram on their rigs.

Another question I have is if my 4 cores are working at 25% instead of 100% would a faster computer really be faster in the same situation?

Thanks