|"can you just stick any old RAM into your PC and it will work?"|
NO. There are several variables related to the motherboard that dictate which RAM you can use, for example: if your board requires DDR3 RAM & you have DDR2, you can't use it. If you have the correct RAM type & your board only accepts up to 1GB per slot but you have a 2GB stick, you can't use it. And assuming you have the correct RAM for the board, the proper RAM speed depends on the CPU. For example, if the optimal RAM speed for your CPU is DDR2-800 but you only have DDR2-533, it should work, but the system performance will be slightly slower than if you had used DDR2-800.
Beware gaming RAM or performance RAM, many times it is just factory overclocked slower RAM. The tell-tale sign is the voltage. DDR RAM is suposed to run at 2.5v, DDR2 at 1.8v, DDR3 RAM at 1.5v. If the RAM specs the voltage higher than that, it should be avoided. For example, this DDR2-800 voltage is spec'd out at 1.8V:
But this DDR2-800 volatge is spec'd out at 2.0v. Notice the timings are exactly the same:
You don't gain anything by getting the 2.0v RAM, in fact, it may cause problems, especially in an OEM system (Acer, Dell, Gateway, HP, etc) that doesn't allow manual adjustment of the RAM settings.