Just because the machine sees the 8GB doesn't mean it will use all of it. The problem many were having is when you got above the threshold of what the machine is able to use (either 4 or 6GB) the rest would go to swap.
I really don't know what you guys are doing to use more than 4GB. I very rarely see over 33% memory usage, and I think the highest I have ever seen is 50%. If I upgraded is would be just to geek out and not because I actually needed it.