No question: 8GB.
You haven't seen any usage above 4GB on your 8GB system because the system doesn't keep track of how much system memory your graphics card takes up when gaming. Yes, your graphics card takes system memory while gaming, using it for texture storage.
To find out how much system memory your graphics card uses, go to the WEI page and click on the "View and print detailed performance and system information" link on the right side.
Total available graphics memory: 4095MB
Dedicated graphics memory: 1280MB (from my GTX 570)
Dedicated system memory: 0
Shared system memory: 2815MB
The "Shared system memory" is the important number. If I only had 4GB of system RAM, it wouldn't be able to have anywhere near as much shared system memory for the graphics card to use. That would mean more graphical anomalies like missing textures, plus poorer game performance overall.
Read this also: Memory-Hungry Graphics Cards?
The parts in German are the detailed performance and system information I showed you how to look at just above in this post.