I build desktops primarily for myself but sometimes for friends and family. I have been building one pretty much every year. I have noticed that the rate of CPU advancement has slowed considerably. I am still using a 4-year old dual core processor. It is behind the technology curve now, but still enough for today's needs.

In general, encoding audio and video benefits from more cores, whereas games demands higher clock frequency.

On the CPU, I would go for Athlon II x4 or Phenom II x4 quad core and pass the Phenom (1) and dual core processors. The difference between Athlon II x4 and Phenom II x4 is the latter has additional L3 cache for a bit more.

RAM wise, 4GB is plenty for now. It's best to get 2x 2GB modules so you will have 2 empty slots for future expansion. Do get the computer with 64-bit OS like Windows 7 x64.

Hard disk should be as much as you need/afford. They are cheap now, but don't skimp on a backup strategy especially when you have a lot of data at stake.

Since the intended user is doing light gaming, the on board graphics should be able to handle it for now. Since you are looking at AMD processors, the preeminent chipset will be 780G, 785G or 890GX.

Should the gaming demand more graphics power in the future, it is not hard to install a midrange GPU (ATI HD5600 or Nvidia GTX260) for $100~$200 that gives you many times of the power of the onboard graphics. The low end don't have enough power to make a difference, whereas high end suffers from diminishing returns.