My RAM usage on Windows 7 runs at about 20% at idle. I have 4 gigs installed, of which only 3.3 is usable because I use 32-bit Windows.
I am using AVG Free 9.0 with Win 7 without any issues.
I seriously doubt Malwarebytes will interfere with MSE or AVG.
Opera is lighter than Firefox, but it is not intuitive. I took a look at it for a couple of hours recently and gave up.
Firefox has just released 3.5.4.
I briefly looked at Windows Live Mail and wasn't amused.
Thunderbird Mail is working well for me on Windows 7 and looks very much like Outlook Express.