Upgrade CPU or GPU which one is better
 

Upgrade CPU or GPU which one is better.

between
Q6600 quad core 2.4GHz
8GB memory
GeForce GTX550 1GB

and
i5 2400 quad core 3.10GHz
8 GB memory
GeForce GT210

I am new with CFD analysis. What is difference between using CPU and GPU to process the simulation?

Unless it is a highly specialized code, your simulation will not make much use of the GPU. The primary factors for your simulation speed will be CPU (both general speed as well as cores if you are using parallel code) and RAM (general speed as well as having a sufficient amount)