Fyi next time just choose the edit button underneath your original post.
In response to your question, we need to know more about how you intend to use the system, but I can tell you right off two things:
(1) Yes a C2D is a lot faster then a 4200+ X2, however if you already have 4gb's DDR1 I'd suggest hanging onto it & putting all your cash into other components.
(2) Forget the X1600XT Crossfire & sell the 1600 then put all your available cash into the fastest available video card you can afford... I'd suggest an 8800GTS/GTX because even with a 4200+ it'll smoke X1600XT Crossfire with a 6600 C2D, really it won't even be close.