If you're running the 32-bit version of Windows 7, it will only utilize a maximum of 3.25GB of the RAM. So unless you're running the 64-bit version, the extra 2GB would probably be a waste of money.
This is not an overclocking website. If you're not sure how to overclock your CPU there are several good AMD overclocking websites you can go to and get advice.
Overclocking can be complicated and tricky if you're not sure what is you're doing.
The 5700 series of Radeon cards has more stream processors than the 5500 series. With gaming, it's important to have as many stream processors as affordable.