Second add in card of most value: PPU or SLI GPU?

[Dave B(TotalVR)]

Its gotta be SLI, coz being as we are talking in the future here with WGF 2 and GPU's having disctinct ALU's on them, its going to be just as effective doing pyhsics on the second graphics card as on the PPU, plus you can get extra fillrate off the SLI card when the load is low.