amd a series will handle all casual games. i dont think you needs a K-series CPU, however. try the amd a4-3400. its at a really good price here: http://www.amazon.com/dp/B006DLP7BM/?tag=pcpapi-20
if you plan to get a graphics card later, do some research on amd's website to see if the graphics card you want is supported by an amd a-series cpu. if the card you chose is more high-end than those that the a-series cpus support (which are the Radeon HD 6450, 6570, and 6670 to A4, A6, and A8 respectively), an AMD Phenom II X4 would be the best choice.