Have never really played with SLI so just thinking out loud - Have always bought a card with the intention of buying a second later, then just upgraded to a single next generation gpu.
I just see the PPU option as being hellishly expensive for what you get, and can't believe that they really expect us to only expect future performance upgraded only through drivers with minimal hardware options... I don't think I can cope with trying to maintain decent gfx performance at the same time as physics.
My problem is that I absolutely love Ghost, and anything that could improve an already awesome game is worth the money in my book.....