AMD's Athlon processors may be old news, but they're also one of the longest lasting, most popular product lines to ever hit the CPU market. If you've got one of these old soldiers and want to give it a fighting chance against today's bigger, meaner processors, then look no further.
Athlon's X2 processors are old. The architecture they're based off was first introduced not long after the turn of the century. But AMD has milked a lot of life out of its old architecture by gradually increasing clock-speeds, lowering thermal footprints, and lowering prices. Even today, Athlon's entire X2 line-up remains a strong contender in the sub-$100 dollar processor market, particularly when AMD's better low-end motherboard chipsets are considered. If you have less than $100 to spend on a motherboard, it's hard to find a good Intel board, but it will take you into the mid-range of AMD's boards.
The downside to the Athlon X2 is that if you currently have one, you may find yourself at the end of the road, in terms of upgrades. Even the fastest Athlon X2 processors do not compete well with true performance processors like the Phenom II and Core 2 - nevermind Core i7. And unfortunately, many older Athlon X2 processors sit on motherboards that don't support the best Phenom processors due to the high power requirements.
If Phenom II would take a whole new motherboard, getting the most from your Athlon is crucial. This, of course, is where overclocking steps in. Athlon X2 processors can certainly achieve overclocks of 25%, if properly cooled. Also, if you have a low-end Athlon X2, upgrading to a "Black Edition" processor may be a good idea, as those processors can be overclocked substantially and feature an unlocked multiplier, which I will explain later.
A warning - overclocking always runs the risk of damaging hardware, and overclocking also voids the warranty on your processor. Keep these risks in mind when overclocking your processor.
The only thing that you must, without exception, have when overclocking is a motherboard that includes BIOS options for overclocking. This includes the ability to adjust the Reference Clock, the CPU Multiplier, the Hypertransport Multiplier, the Northbridge Multiplier, and the Memory Multiplier. You do not necessarily need access to all of these options in order to overclock, but you at least need to be able to adjust the Reference Clock, and access to all of these options is required to reach the highest overclock possible.
Determining if your motherboard has a BIOS which allows these changes can be done either by consulting the manual of your motherboard or by exploring your BIOS. Most BIOSes which have serious overclocking support are going to lump these controls in a specific category, which can be accessed from the main BIOS menu. Unfortunately, there is no standard which says what these controls must be called, so having a motherboard manual to reference is useful.
There are no other requirements for overclocking. For reaching a maximum overclock, having a processor with an unlocked CPU multiplier, RAM with room for overclocking, and a good cooling solution are all good idea. However, if you don't already have these items, you shouldn't buy them unless you're more interested in reaching the highest overclock possible then getting the best value out of your current hardware.