I'm happy with 8
I went round the houses on this for ages...
Originally ordered 2.6,512,8 because I was planning to upgrade memory at a later date. That one is going back as unless you need the extra drive space its kind of the odd one out in upgrade terms.
I then ordered the 2.6,512,16 but considered the price and thought for £2450 it was too much for my first mac! The processor speed bump is minimal, the extra drive space something I can easily manage to live without and the 16GB of memory could only be theoretically used by me anyway. That was cancelled before it shipped.
I then went to my local store and picked up a 2.3,256,8 and in the real world it's perfect for my needs. I've saved a good chunk of cash and looking at my activity monitor the 8GB is plenty. Performance is superb and of course Diablo 3 plays nice (JOKE). I haven't used 4GB yet so I'm confident the system has plenty more capacity.
Unless you are going to use more than 1 VM or edit large amounts of large video or raw files then 8GB is already overkill IHMO.
As an aside, I think there should be more emphasis placed on how poor the retina screen looks under Windows 7 and how badly the trackpad appears to work? I installed the 64BIT version via bootcamp and was very disappointed. It certainly wouldn't be an environment I'd want to use a lot!
Just my 2 pennies.