Crysis: Warhead is not a particularly optimized game and Crysis is even less so.
If you are looking for everything maxed out with something close to a minimum of about 30 FPS, then I would recommend at least two HD 4890 XFired or two GTX 280 SLIed.
A single card solution with two GPU on it would be the GTX 295.
If you simply want a single card with a single GPU, then it would be the GTX 285 or HD 4890, but maximum FPS will be less than 30.
Those benchmarks are based on a system built around an Intel Core i7-965 Extreme Edition processor (3.2GHz). Therefore a less powerful system will likely have slightly lower FPS.