As pinhedd said, your hardware is quite old and probably causing your performance issues... certainly in Crysis....
Dirt 2 I would expect to run okayish.... you dont have much ram.... but don't really wanna say that getting more would help you a great deal... I would be using 4gb if I were you anyway though....
If you turn the settings to medium do things get any better?
turning down the settings does not help either as the problem still persists. I do suspect that this could be because of low ram or a weak PSU
Could you suggest what would be more preferable?