The amount of speed boost you will get from running raid drives probably isn't quite as much as you would get from buying a better processor. IMO, a 4200+ is a little low end if you are wanting to get a more updated PC. That is barely faster than my last PC (that my server is running on atm). Keeping the raid is definitely nice if you can afford it though and a 6400+ AMD isn't really that much more expensive and you get 50% more speed over a 4200+. Normally, when I am CPU shopping, I look at the prices and you can almost see a perfect scale of speed vs price. At a certain point, you will see that the amount of speed increase you are getting per dollar will drop quickly. Normally that is in the <200$ range. After that point, they start to be minor upgrades for a considerably higher price. So, just look for the best bang for your buck.
I normally use
http://www.sharkyextreme.com/ to find the latest price ranges for CPUs and it is a quick way to decide which is the best for you.