Thanks for answers. It is not obligatory for Crysis (not to say Crysis 2) to run on max setting just to run decently on med-high settings.
As for mobo I have this one.
It supports only DDR2. I guess I could put some older Phenom on it but I'm not sure if it's actually worth it. I think that Athlon II and Intel's i5 are more popular right now.