Crysis Warhead uses the CryEngine 2 not the CryEngine 3.
Only Crysis 2 uses the Cry Engine 3 so that should be the "starting point".
A Q6600, 4GB of Ram and a GTS250 should have no problems running the game using DX9.
Was just about to type what this guy said. Crysis 2 should be your starting point, NOT Warehead. Cryengine2 is actually LESS optimised so will probably run worse than given the same settings and isn't a good benchmark. That said, I will assume that MWO will utilise DX11 out of the box (unlike C2) so you should judge your rig based on max settings and take it from there. - especially if its going to use tessellation (fingers crossed!).
Im running on a 560ti and it runs C2 pretty well on max settings, 1080p. They can be had fairly cheap now, but if I would say one thing is that 1GB RAM on the card simply isn't enough - this card regularly runs into memory issues once AA is introduced.