To answer your question about whether 8gigs will be used or not depends on you. I have maxed out the 8gigs and I monitor my Acitivity Monitor quite frequently. An example of where I have had all 8gigs come into play was when my system had OSX running with Windows 7 in VMware Fusion running 3 or so programs, not light ones either, and over in OSX I ran Photoshop CS5 with a RAW image and 10+ tabs in Google Chrome. My computer fluctuated around 6-8gigs of ram usage through out the entire time.
I used this as a guideline for upgrading my Macbook Alum to 8GB and things seem to be running smoothly for the past day or so.
I'll report back if there are any issues.