Your slowdown is most likely multidimensional. How much ram do you have installed?
An old platter based hard drive can slow down when you ask it to access too much data at the same time.
Finally, if all your doing is web stuff and general office type tasks, a decently clock dual core will be plenty. so the i5 650 or even an i3 processor will do the trick.
What do you have now for system specs?