This also depends on your CPU. You'll be wanting a good quad-core CPU.
I don't know of any system that can play Crysis with the ABSOLUTE highest resoltution and highest settings but a good quad-core and 2xHD4850 will provide a very, very nice gaming experience.
It's also important to keep the frame rates high. Assuming the rest of your system has similar specs, I recommend running the Crysis benchmarks or simply running FRAPS and determine your frame rate then set settings according to be at LEAST 40FPS at all times.
If possible, it's nice to be running at 60FPS constantly using VSYNC. VSync eliminates any screen tearing and also reduces the computer from generating frame rates above 60FPS (which generate heat but do nothing for the screen which can't use above 60FPS).
Older CRT monitors often have refresh rates of 75 or 85Hz.