The cheapest RAM is value RAM, that's intended for non-overclock use. However, This memory can still be overclocked 'a little bit'. This memory mostly run at default speed. (800Mhz for DDR2), which is quick enough. This uses chipset Elpida, Promos, Infineon, etc.
The best RAM is Extreme RAM, that's intended for achieving maximum performance through overclocking memory. This memory is mostly paired with extreme CPU and motherboard. This RAM is usually made of Micron chipset. A very good chip but also a more expensive one. This RAM can run faster than normal memory standard speed in its class (Example, 1066Mhz for DDR2). But also requires tweaking for overclocking.
Quality brands are A-Data, Corsair Dominator, albeit almost twice as expensive. Team Extreem, Patriot, OCZ, G.Skill, Kingston. They also all have lifetime warranty. Quality brands.