If you are purely looking for the best bang for the buck in the ATI graphics card lineup then the HD 4650 and HD 4670 are definitely the best choice. While their 3D performance can certainly not rival high-end cards they are good for some pretty serious gaming.
What makes the Sapphire HD 4650 OC Edition very interesting is the enormous overclocking potential left in the card. Even though Sapphire uses a rather small cooler, we could overclock our card by more than 20% - free extra performance! Unfortunately Sapphire's thermal settings in the BIOS are not nearly as silent as other HD 4650 models. In idle the fan is very well audible even through a full case. Under load the fan ramps up in speeds even more resulting in noise levels similar to the NVIDIA GTX 260, which is more than twice as fast. This can be easily fixed though by tweaking the fan settings in the BIOS with RBE to the noise/temperature balance that you want.