I just bought today 3x2GB 1600 and add to my old 3x1GB 1600 on my i7 920 like yours.
I don't know how to test this solution scientifically but my Win7 performance index for the ram jumped from 5.9 to 7.5.
I made two antivirus analysis (AVG and Windows 7's) on my whole computer : 3 Raptor HDD (75 GB/150 GB/75 GB), the performance gain is quite impressive. It went from 20 minutes (AVG) / 10 minutes (Win7's) to 5 minutes (AVG) / less than 2 minutes (Win7's).
To perfect my installation, I'm just wondering if 3x1 on the first slot is better than 3x2 on first slot...