I wouldn't bet on Crysis running on a normal CPU/GPU setup, at least not with all the feature turned on. Their recommended hardware is dual-core with Vista (and I wouldn't be surprised if their demo machine ran something larger).

And, while I am impressed with Crysis, I've seen better physics demos and better graphics demos (not in a game, just on their own). Crysis is good, but it could be even faster and more stable if they had more "clocks".

As far as "just adding a second GPU", that's an expensive solution. If the average consumer could choose between a second GPU at $500 or a PPU for $250, the choice is clear. Even better if they could pay $100 extra and purchase a video card with an extra PPU chip on it.